Basically, I want to be able to use it as a mic and/or use the button tones as an instrument. preferably in one unit, but if two phones have to be used, so be it.
are we talking about a mobile phone here or a normal one?
Theres some information in the links section on connecting a telephone mic to a line level source but what i'd suggest you do it just get one of those boxes that you plug the telephone jack into that are designed for recording telephone conversations.
buttons can be used to control audio matrix, ie columns and rows. inputs on columns outputs on rows. make feedback connections between cross points. outs to main/auxes/effects/column ins. see nic collins harware hacking book for more on audio matrices. trickier with mobiles because of surface mount/minaturisation. :#
personally i'd suggest that a microwave might be fairly near the top of my list of stuff not to mess with if you don't happen to be a qualified service engineer.
However, having looked into just how terrifyingly explosively murderous microwaves are when fat fingered idiots like me start rummaging round inside them, I'll be leaving them alone.