Howard Harte hacked the connection between an Astrix PBX and old payphone by emulating the coin line.
This project creates the high voltage signals a Western Electric 1D/2D Payphone requires to manage calls, accept coins, etc. These signals include both +130VDC (coin collect) and -130VDC (coin refund) as well as +25VDC for the “stuck coin” test and -25VDC for the “initial rate” test. It can also count dial pulses to provide out-of-band signalling to Asterisk for the rotary-style 1D1 payphones.
Additional details on the project, including the code, can be found on this GitHub page.