The answer if its possible is:
-probably yes for android and
-no for iOS
Here is a video for acoustic modem using speaker and mic http://www.youtube.com/watch?v=bLRK51_1L2k
I guess it would be possible to make Android app that would receive call and demodulate incoming data.
As for the iOS - the problem is in their Terms Of Service.
Because Apple don't want anybody to record phone calls (to obey the anti wire-tapping laws),
it dosn't allow to forward call to app.