While there are other feature groups for local access,[1] the four common feature groups exist for access from the local subscriber to competitive long-distance carriers:
Feature Group A
The original implementation, in which a user has to dial the local telephone number of a provider's gateway, followed by (usually) a password, then the desired long-distance number. There is a different local access number in each local calling area. This requires no special capability at the local telephone company office as competing long-distance providers connected using standard local lines, which may or may not support caller ID. If a carrier has no local presence, a foreign exchange line is used to reach its nearest point of presence.[2] Once the standard means of accessing alternate long-distance carriers, local access numbers are now used primarily for low-cost prepaid calling cards as the calls may be made from any phone, at flat or local rates.
Feature Group B
Associated with 950-XXXX calling; instead of a local telephone number the user enters 950 and 4 additional digits which identify the long-distance carrier. Operation is similar to the local access numbers (feature group A) except that the 950-XXXX access number is the same in every community, NANP-wide.[3] Some exchanges send the caller's number automatically; where this service is not provided or not desired (calling card applications), the 950-XXXX number must be followed by a calling card number and the long-distance destination number. If ANI is provided, calls from the one subscribed line may be made as 950-XXXX and the long-distance destination. Largely deprecated by feature group D, but the 950 prefix and a list of carrier codes[4] remain reserved in all North American area codes, even in Canada where most providers went from feature group A directly to 1+ default carrier dialling and feature group D (101xxxx + destination) calling without ever using 950-XXXX as a primary means to access alternate long-distance carriers from home land lines.
Feature Group C
Rare, originally used by AT&T for operator-assisted coin phones since they allow the operator to keep control of the caller's telephone line until the transaction is completed.[5] As coin-handling for trunk calls is now automated within the phone (like a COCOT, the current generation of coin phones operates self-contained without the central exchange providing coin-call support functions), group 'C' is largely obsolete.
Feature Group D
The current standard, requires the local switch support equal access by competing carriers at the trunk level; highest quality connection, and allows pre-selection of the interexchange carrier by the end-user. This feature group permits two types of calls. If a user dials 1 + area code + seven-digit number, the long-distance call is handled by a default carrier chosen by the user. Alternatively, a user dials 101 + four-digit carrier code + area code + seven-digit number, and the call is handled by the carrier specified by the carrier code.[6] The original batch of carrier codes began with 0, so this type of "dial around" service was typically marketed as dial-around 1010-xxx service. NANPA maintains separate lists of carrier codes for feature groups 'B' and 'D'[7] as not all long-distance providers support both standards.
These Feature Group alternatives allowed the LEC's end users to make long-distance calls using the interexchange carrier's network, when non-stored program-controlled exchanges could not be modified to provide equal access. By the mid 1990s, Equal Access features in exchange software had rendered Feature Group D universally available in modern landline exchanges; the others are either used for calling card applications or are obsolete.
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