You should probably start with one of these then add an MP3 Player shield. These guys are great for parts and also do a really great cheap custom PC board if you are not in a hurry. 
The interface to the phone line should be pretty simple. I would start with something like the following circuit to interface all the parts. The 470 and 100uf and isolation transformer are probably optional, but the 1k might be nice to limit the volume. I think the off hook dial tone is typically 400hz. You could inject this tone when you detected a voltage change on the analog in due to the phone going off hook. Listen for a dial tone sequence with the MT8870 and then turn the tone off and play the song indicated by the tone sequence.
Good Luck...
