The Telephone XLR8OR is a simple circuit that allows you to connect an old analogue telephone to any mixer or microphone preamp that can supply 48V phantom power and use it as a microphone.
It works by taking the 48V phantom power present on the two balanced signal lines of the XLR connector (pin 2 and 3) via the resistors R1 and R2 and connecting them across the phone lines using the two 100Ω resistors R3 and R4.
This causes two out of phase signals to appear at each resistor which can be coupled back into the balanced signal lines via the two capacitors C2 and C3.
Since the impedance of the phantom power supply is fairly high the power delivered to the phone might not be enough for all telephone functions, but at least for my phone audio and touch tone dialing works fine.
I designed a small circuit board in KiCAD whose silk screen and copper layer I printed out and glued onto a piece of protoboard to use as a guide during assembly.
Finally I 3D printed a small enclosure for it that would fit the circuit board, an XLR plug and an RJ11 phone jack.