I built an audio 4 connector 3.5mm splitter. The goal is to play a sound clip through a phone's mic input across a call, while recording the same sound clip on the computer. I used the TRRS configuration of: Tip = Left audio, Ring = Right audio, Ring = Ground, Sleeve = Mic. This is apparently the configuration for an Apple headset. The recording part works on most phones, but the playing of a sound clip into the mic input of the phone only works on a select few. By the way, I am connecting speaker output on the one side to the mic input on the other side.
I realized that each phone has different impedance requirements. How do I adapt the cable to work on any phone? When I do impedance matching, how do I get to the total value of eg. 23 Ohm specified in the technical specifications of headsets? Is there something I can buy that will do the impedance matching for me or should I build it myself?