Hi, im working on this (fairly simple) project with an old dial phone i had laying around. Im trying to make it work as a normal wired headset, connected to my smartphone through a 4-pin aux cable.
So far i have been able to get it to play music through its original speakers, but I'm still having trouble with the microphone. I know where the microphone pin is on my aux cable, but it still won't work.
I also know that the microphone on the dial phone works, because I have cut open an old 4-pin headset I had laying around, and rewired the dial phones microphone to it's microphone.
Despite every component working as it should, I'm still not able to get the mic to work, without worrying it through anothr pair of headphones first (which seems like a silly solution.)
Is there anything I missed? Maybe a special circuit that gets the phone to recognize the dial phone as a microphone?

Btw, I'm not using the old circuit that was in the dial phone, so it's only wires, a speaker, a microphone and a 4-pin aux cable.

 

Headset or handset ? There is old telephones with carbon microphones and later, dynamic microphones, and less old with electret microphones in their handsets. What do you have and how does it terminate ? In a RJ-22 / 4P4C plug ?



Then you need to know your smartphone jack pinout configuration ; there is at least two types of TRRS.
----> https://components101.com/connectors/35mm-audio-jack
----> https://connector.pinoutguide.com/4_pin_3.5mm_2.5mm_plug/

I use a modern handset terminated in two 3.5mm plugs to fit my laptop mic jack and ear jack for 20 years now.

 

I am suspecting that the "old dial phone" predates the modular connectors by quite a bit. many of those MUCH EARLIER phones had a 3-wire handset cord, as well as a carbon microphone.
While both a carbon mic and an electret mic require bias voltages, the voltage is a bit different and the current is quite a bit different. The carbon mic requires more current