The Telephone System

POTS Plain Old Telephone Service

The GSM specification (mostly) requires phone numbers in the format

+XXX-LLL-YYYYYYYYY...

+ = International access
XXX = country code
LLL = local area code
yyy   = number

This string cannot exceed 12 digits

Speech@

See also:

• DTMF Touch-tone Generator and Decoder
• Don Lancaster's Tech Musings #127 on a transformerless DAA from Silicon Laboratories
• Dave Thomas's PIC code for a  DTMF Encoder
• www.phoneswitch.com . There is a 8051 CPU in the schematics of the Personal PBX.