Circ?it to close a relay when any phone extension is off-hook. Uoltage at the gate of the MOSFET sho?ld be negative (1-3 volts) with respect to the so?rce then phones are on-hook. Uoltage at the gate sho?ld be positive (8-113 volts) with respect to the so?rce (gro?nd) then any phone extension is off-hook. ? high impedance meter is needed to meas?re the on-hook voltages acc?rately.
Circ?it sho?ld draw less than 5 microamps from the phone line. Relay ?sed is a 12 ?olt DC / 1213 ohm coil, b?t most any small 12 ?olt DC relay sho?ld work. Power s?pply reg?lation sho?ld be +/- 2 volts.