check this
8) Hello, for your porpouses, you can find numerous dtmf dialers from some manufactures, but may be hard to get them. Instead if you have a bit litte control on uC, you can use a small PIC, and the MT8870 DTMF tone decoder. So, you use the pis as tone dialer anf MT8870 as tone decoder to make a complete telefon dialer.
Be sure, it will be much easy and cheap than buy a tone dialer
Chek this file, it's a complete tono dialer/decoder, with a pic and source code
I hope you be usefull