See also
- Follow-me
- Hunting
- Unstructured Supplementary Service Data - list of standard GSM codes for network- and SIM-related functions
Call forwarding, or call diversion, is a telephony feature of all telephone switching systems which redirects a telephone call to another destination, which may be, for example, a mobile or another telephone number where the desired called party is available. Call forwarding was invented by Ernest J. Bonanno. [1]
In North America, the forwarded line usually rings once to remind the customer using call forwarding that the call is being redirected. More consistently, the forwarded line indicates its condition by stutter dial tone. Call forwarding typically can redirect incoming calls to any other domestic telephone number, but the owner of the forwarded line must pay any toll charges for forwarded calls. Call forwarding is often enabled by dialing *72 followed by the telephone number to which calls should be forwarded. Once someone answers, call forwarding is in effect. If no one answers or the line is busy, the dialing sequence must be repeated to effect call forwarding. Call forwarding is disabled by dialing *73. This feature requires a subscription from the telephone company. Also available in some areas is Remote Access to call forwarding, which permit the control over call forwarding from telephones other than the subscriber's telephone. VOIP and cable telephone systems also allow call forwarding to be set up and directed via their web portals. Call forwarding can be Conditional or Unconditional. Conditional call forwarding only works when the conditions set by the customers met while Unconditional call forwarding works in all cases irrelevant of network coverage.
In Europe, most networks indicate that unconditional call forwarding is active with a special dial tone. When the phone is picked up it is immediately apparent that calls are being forwarded, while in other countries same system is being followed now.
The ISDN Diversion supplementary services [2] standards document uses "diversion" as a general term to encompass specific features including "Call Forwarding Busy", "Call Forwarding No Reply" and "Call Deflection".
The terms call forwarding and call diversion are both used to refer to any feature that allows a call to be routed to a third party, and the terms are generally interchangeable.
Special types of call forwarding can be activated only if the line is busy, or if there is no answer, or even only for calls from selected numbers. In North America, the North American Numbering Plan (NANP) generally uses the following vertical service codes to control call forwarding:
| Forward service | Activation | Deactivation | Number change |
|---|---|---|---|
| All calls | *72 *72[1+e number] (some larger cities) | *73 | |
| On busy or no answer | *68 | *81 | |
| From select callers | *63 | *81 | |
| On ISDN | *56 | ||
| Customer-programmable features (where available): | |||
| Busy line | *90 | *91 | *41 |
| No answer | *92 | *93 | *611 |
The Sprint Nextel cellphone company uses these: [3]
| Forward service | Activation | Deactivation |
|---|---|---|
| On busy or no answer | *28[phone number] | *38 |
| On busy | *74[phone number] | *740 |
| On no answer | *73[phone number] | *730 |
| Immediately | *724704732775 | *720 |
Most EU fixed-line carriers use the following codes based on CEPT and ETSI standards developed in the 1970s on both POTS and ISDN lines. (There may be some variation to these, but the unconditional code *21* is very much universally standard on EU telephone lines.) The general syntax for all European service codes always follows the pattern below:
| Forward service | Activate | Cancel & Retain | Reestablish | Status |
|---|---|---|---|---|
| If busy | *69*[phone number]# or *67*[phone number]# | #69# or #67# | *69# or *67# | *#69# or *#67# |
| If not answered | *61*[phone number]# | #61# | *61# | *#61# |
For GSM/3GSM (UMTS) phones, the GSM standard defines the following forwarding Unstructured Supplementary Service Data. These were developed by ETSI and are based on standard European diversion codes and are similar to those used on most landlines in the EU: [6]
| Forward service | Activate | Cancel & Deregister | Cancel & Retain | Status | Reestablish |
|---|---|---|---|---|---|
| Unconditional forwarding | |||||
| All calls | *21*[phone number]# | ##21# | #21# | *#21# | *21# |
| Conditional forwarding | |||||
| If busy | *67*[phone number]# | ##67# | #67# | *#67# | *67# |
| If not answered (see below for custom delay) | *61*[phone number]# | ##61# | #61# | *#61# (incl. time delay) | *61# |
| If out of reach | *62*[phone number]# | ##62# | #62# | *#62# | *62# |
| All forwards | *002*[phone number]# | ##002# | #002# | *#002# | *002# |
| All conditional forwards | *004*[phone number]# | ##004# | #004# | *#004# | *004# |
If the prefix to the forwarding command is "**" (instead of the usual "*"), then the phone number in that command is registered in the network. If after that the forwarding is deactivated using a command with a single "#", then later it will be possible to re-activate this forwarding again with a simple "*" command without a phone number in it. The forwarding will be re-activated to the number registered in the network. For example, if one uses the out-of-reach code in a forwarding command:
**62*7035551212#
and after that one deactivates the forwarding:
#62#
then later it will be possible to re-activate the out-of-reach forwarding without specifying a number:
*62#
After the above command, all calls made to the phone, while it is out of reach, will be forwarded to 7035551212. It is possible to activate the feature to a number other than the registered number, while still retaining the registered number for later use. For example, issuing the command:
*62*7185551212#
will result in calls being forwarded to 7185551212 (and not to the registered number 7035551212). However, if later a command is issued:
*62#
then the calls will again be forwarded to the registered number 7035551212 (and not to the number from the previous forwarding command 7185551212).
In GSM networks of some US carriers, and in all mobile networks in Europe, it is possible to set a number of seconds for the phone to ring before forwarding the call. This is specified by inserting "*SC*XX" prior to the final "#" of the forwarding command, where "SC" is a service type code (11 for voice, 25 for data, 13 for fax), and "XX" is the number of seconds in increments of 5 seconds. If "SC" is omitted (just "**XX") then by default all service types will be forwarded. For example, forwarding on no-answer can be set with:
*61*[phone number]**[seconds]#
Forwarding voice calls only can be set with:
*61*[phone number]*11*[seconds]#
In some networks there may be a limit of not more than thirty seconds before forwarding (i.e. “XX” can only be 05, 10, 15, 20, 25, or 30; all greater values, like 45 and 60, will result in the forwarding command being rejected and an error message returned).
Diverting calls can increase one's availability to callers. The main alternative is an answering machine or voicemail, but some callers do not wish to leave a recorded message, but want to have a two-way conversation.
Some businesses have their calls forwarded to a call center, so that the client can reach an operator instead of an answering machine or voice mail. Before the availability of call forwarding, commercial answering services needed to physically connect to every line for which they provided after-hours response; this required their offices to be located near the local central exchange and be fed by a huge multi-pair trunk in which a separate pair of wires existed for each client subscriber. With call forwarding, there is no physical connection to the client's main telephone service, which is merely call-forwarded to the answering service (usually on a direct inward dial number) at the end of the business day.
Often, a suburb of a large city is a toll call from many suburban exchanges on the opposite side of the same city, even though all of these suburbs are a local call to the city centre. A business located in such a suburb may therefore benefit from obtaining a downtown number as an "extender", to be permanently forwarded to their geographic suburban number.
Where unlimited local calls are flat-rated and long-distance incurs high per-minute charges, the downtown number's wider local calling area represents a commercial advantage. Markham (directly north of Toronto) is long-distance to Mississauga (directly west of Toronto). A Markham business with a forwarded 416 number could receive calls from Toronto's entire local calling area without incurring long-distance tolls (as both legs, Mississauga → Toronto and Toronto → Markham, are each a local call).
Some services offer international call forwarding by allocating for the customer a local virtual phone number which is forwarded to any other international destination. The number was permanently forwarded and had no associated telephone line. As a means to obtain an inbound number from another town or region for business use, remote call forwarding schemes tend to be far less expensive than foreign exchange lines but more costly than using voice over IP to obtain a local number in the chosen city.
Call forwarding can also assist travelers who do not have international cell phone plans and who wish to continue to receive their voicemails through VoIP easily while abroad.
Digital Enhanced Cordless Telecommunications (DECT) is a cordless telephony standard maintained by ETSI. It originated in Europe, where it is the common standard, replacing earlier standards, such as CT1 and CT2. Since the DECT-2020 standard onwards, it also includes IoT communication.
The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation (2G) digital cellular networks used by mobile devices such as mobile phones and tablets. GSM is also a trade mark owned by the GSM Association. "GSM" may also refer to the voice codec initially used in GSM.
General Packet Radio Service (GPRS), also called 2.5G, is a mobile data standard on the 2G cellular communication network's global system for mobile communications (GSM). Networks and mobile devices with GPRS started to roll out around the year 2001. At the time of introduction it offered for the first time seamless mobile data transmission using packet data for an "always-on" connection, providing improved Internet access for web, email, WAP services, and Multimedia Messaging Service (MMS).
QSIG is an ISDN based signaling protocol for signaling between private branch exchanges (PBXs) in a private integrated services network (PISN). It makes use of the connection-level Q.931 protocol and the application-level ROSE protocol. ISDN "proper" functions as the physical link layer.
The Telephony Application Programming Interface (TAPI) is a Microsoft Windows API, which provides computer telephony integration and enables PCs running Microsoft Windows to use telephone services. Different versions of TAPI are available on different versions of Windows. TAPI allows applications to control telephony functions between a computer and telephone network for data, fax, and voice calls. It includes basic functions, such as dialing, answering, and hanging up a call. It also supports supplementary functions, such as hold, transfer, conference, and call park found in PBX, ISDN, and other telephone systems.
An emergency telephone number is a number that allows a caller to contact local emergency services for assistance. The emergency number differs from country to country; it is typically a three-digit number so that it can be easily remembered and dialed quickly. Some countries have a different emergency number for each of the different emergency services; these often differ only by the last digit.
Caller identification is a telephone service, available in analog and digital telephone systems, including voice over IP (VoIP), that transmits a caller's telephone number to the called party's telephone equipment when the call is being set up. The caller ID service may include the transmission of a name associated with the calling telephone number, in a service called Calling Name Presentation (CNAM). The service was first defined in 1993 in International Telecommunication Union – Telecommunication Standardization Sector (ITU-T) Recommendation Q.731.3.
The Hayes command set is a specific command language originally developed by Dale Heatherington and Dennis Hayes for the Hayes Smartmodem in 1981.
A busy signal in telephony is an audible call-progress tone or audible signal to the calling party that indicates failure to complete the requested connection of that particular telephone call.
Call waiting is a telephone service where a subscriber can accept a second incoming telephone call by placing an in-progress call on hold—and may also switch between calls. With some providers it can be combined with additional features such as conferencing, call forwarding, and caller ID. Call waiting is intended to alleviate the need to have more than one telephone line or number for voice communications.
A vertical service code (VSC) is a sequence of digits and the signals star and pound/hash dialed on a telephone keypad or rotary dial to access certain telephone service features. Some vertical service codes require dialing of a telephone number after the code sequence. On a touch tone telephone, the codes are usually initiated with the star key, resulting in the commonly used name star codes. On rotary dial telephones, the star is replaced by dialing 11.
The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists of telephone lines, fiber-optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables interconnected by switching centers, such as central offices, network tandems, and international gateways, which allow telephone users to communicate with each other.
Unstructured Supplementary Service Data (USSD), sometimes referred to as "quick codes" or "feature codes", is a communications protocol used by GSM cellular telephones to communicate with the mobile network operator's computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network. The service does not require a messaging app, and does not incur charges.
A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX).
GSM services are a standard collection of applications and features available over the Global System for Mobile Communications (GSM) to mobile phone subscribers all over the world. The GSM standards are defined by the 3GPP collaboration and implemented in hardware and software by equipment manufacturers and mobile phone operators. The common standard makes it possible to use the same phones with different companies' services, or even roam into different countries. GSM is the world's predominant mobile phone standard.
On analog telephone lines with special services, a flash or register-recall signal is used to control functions on the public telephone exchange, PBX or VoIP ATA.
Malicious caller identification, introduced in 1992 as Call Trace, is activated by the vertical service code *57, and is an upcharge fee subscription service offered by telephone company providers which, when dialed immediately after a malicious call, records metadata for police follow-up. Generally, law enforcement will only act on the trace once a formal police report has been filed in regard to the call.
Numbers on the Irish telephone numbering plan are regulated and assigned to operators by ComReg.
The Brazilian telephone numbering plan uses a two-digit area code plus eight-digit local phone numbers for landlines and nine digits for mobile lines. Public utility services use short phone numbers, always starting with 1.
A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices for data transmission via the public switched telephone network (PSTN), or other public and private networks. Modern smart phones have added a built-in layer of abstraction whereby individuals or businesses are saved into a contacts application and the numbers no longer have to be written down or memorized.
