Call audio over 3.5mm stereo jack using phones microphone, App possible?

[AeonD]

Have FM transmitter for listening to music from phone in car and would like to also have calls go over car speakers but use the phones microphone. Is there an App available/possible to do this? Have tried a supposed hardware fix, 3.5mm male jack with mic to 3.5mm female stereo Nokia brand, and didn't work. Have found mention of another generic hardware jack on forums here, seidioonline.com. Is this the only option or is there an"App fix" possible or already out there?