On a standard 2.1 mm headset jack the tip is the mike and the ring is the ear piece. A standard 4 pin modular handset cord uses the two inner conductions for the ear piece and the outer conductors for the microphone.
I have done this on several telephones with varying success. The two big 'ifs' are as follows:1) The telephone must expect an electret condenser mike, and yes they are polarity sensitive. If the phone is looking for a carbon mike element or an assembly emulating one, you're out of luck. 2) The design of the phone must be such that the mike the ear piece share a common ground. You have 4 wires and 3 pins, so if you can't common the grounds, its not going to happen. You could modify the headset to use a 4 pin connector, but that defeats the purpose of being able to use the headset as is.
This all assumes that you are talking about a dial-in-base telephone.
If you have the dial-in-handset type, the base is often nothing more than a weighted cradle that passes tip and ring through to the handset, and none of this applies.
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