Thats a complex circuit, I've done just this with a pic a decoder and a dual opamp chip (romans done it without the decoder which is quite clever), I think I could do away with the op amp and use a tranny, mine is for acces to a building.
My circuit doesnt have a phone line interface, re-reading your op you dont either, you just want a mic, if you want I'll see if I can find my schematic, its very sensitive so you can turn the volume down on the fone to protect against it being recorded.
I dont think the firmware would be of much use to you I never bothered with being able to change the code, its burnt into the chips program memory, for a commercial product you'd want it user programmable and stored in the pics eeprom.
Heres an interesting article, I used both op amps in the lf358 this guy doesnt:
**broken link removed**...