Something like this was talked about a few months ago. Maybe it will give you an idea? Have not done this myself, so not much help from me.

If the DTMF at the source was changed to a digital signal, and the receiving end would change the digital to one of the DTMF frequency?

http://www.picbasic.co.uk/forum/showthread.php?t=9568
There are links in the thread to other threads about the same subject.