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As CLASS was an AT&T trademark, the term vertical service code was adopted by the North American Numbering Plan Administration. The use of vertical is a somewhat dated reference to older switching methods and the fact that these services can only be accessed by a telephone subscriber, going up ( vertically ) inside the local central office ...
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.
To activate Remote Access to Call Forwarding, a subscriber calls a provider-supplied Remote Access Directory Number, enters the telephone number of the line to be redirected along with a personal identification number (PIN), a vertical service code (such as 72# or *73) and the number to which the calls are to be forwarded.
This permits rich call-related services to be developed. Some of the first such services were call management related, call forwarding (busy and no answer), voice mail, call waiting, conference calling, calling name and number display, call screening, malicious caller identification, busy callback. [7]: Introduction xx
The test consists of calling a reverting calling telephone number (ringback number) or a vertical service code with a standard telephone. Upon answering the call, the exchange plays a unique signal tone as a signal for the installer to hangup the telephone. The exchange then places a reverting call to the originating line, causing the telephone ...
Extensions that were busy or rang "no answer" would forward to the message center using a device called a "call director". The call director had a button for each extension in the company which would flash when that person's extension forwarded to the message center. A little label next to the button told the operator the person being called.
In linear hunting, calls are always delivered to the lowest-numbered available line. This is also referred to as serial hunting or terminal hunting. It resembles busy/no-answer call forwarding. Calls are always delivered to the first line, unless it is busy—then the second, third, and remaining lines are considered in succession.
In the United States, both interexchange carriers (IXCs) such as Sprint, AT&T Inc., and Verizon, and Local Exchange Carriers (LECs) such as Verizon and AT&T offer toll-free services. [2] [3] The way that a toll-free number is handled depends on whether it is a domestic or an interexchange call.