Telephone and Intercom Schematics 2

Telephone Line Monitor
Use old phones as an intercom
Phone-In-Use indicator
Hold function for Telephone
Digital/Standard Phone Line Tester
Caller ID decoder part 1
Caller ID decoder part 2
DTMF decoder / logger
Cellular Phone calling Detector
Phone line in use indicator
Telephone music on hold interface
Soft musical telephone ringer
Telephone amplifier
Telephone line monitor
Cut telephone line detector
FM telephone line bug transmitter
Simple telephone line tap for intercepting telephone calls
The Link A2B+1 (the Link Telephone Intercom – DTMF version)
The Link 4+0 – Internal Intercom
The Original 2 Phone Intercom Link Design
8-line intercom system using 89C51 microcontroller
Universal telephone hold
Talking phone dial monitor
Telephone line simulator
Telephone privacy adapter
Telephone ringing circuits
Simple circuit to connect telephone equipment to audio mixer
NCDS Interface Schematic for Nokia 3110, 8110, 8110i
Assorted telephone circuits
Greek telecard reader
Phone ring circuit schematic
Cut phone line detector
Telephone autodialler
Taxi phone automatic dialler