What I did:
After figuring out that my phone is alway leaving the internal microphone on (and not turning to "external" microphone aka morserino) I connected an normal phone headset which ... surprisingly worked. Then I measured the resistance of the microphone and put in a resistor for mimicking that.
Don't know about the sound quality though.
If your MacBook also has to decide if there is an external microphone connected or not and you are not receiving anything I guess its worth a try...
Maybe you can doublecheck in the sound setting the changes of connecting normal headphones vs. connecting a headset. If there are automatic changes there I guess you found your problem.