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Many design has used microcontroller to act as dialler. It can use 5 bit or 6 bits port to provide a D-to-A inteface and output as the necessary timing and tolerance. I remember that National Semiconductor has article talking about the software implementation.
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