Wow... this is nice ... looks better than Conexant ,That means we have multiple (almost readymade) solutions to develop an Arduino Telephony base but it seems people are happy with their GSM shield .......... =(
By the way have you noticed it is one of the most viewed thread 
even if you type "Arduino Modem" at google the concept image appears at first place which means it has some sale potential ............
Riva:
Yet another simpler idea like yours is maybe this. It will probably fit into a shield footprint with the few additional components needed like speaker, switches to select serial port pins (hardware/software) and a couple of jumpers to optionally break out signals like RING, CTS.