Yes a good product idea gone dead. I had looked into this in some detail and the next best option I could think of is a shield with telephone sockets and protection circuits ready to receive one of these plug in modules. They do several module types (dial-up, Ethernet & bluetooth) all using the same form factor so maybe the shield could be made universal.I suspect the big problem/cost here is getting approval to connect to your phone line.
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.
BURRAAK, it's been almost a year since your original poste so I was wondering if you've made any progress? I am working on a project in which I am in need of a very similar item. Do you have any leads or suggestions on where I might find information to help me complete this? I have a Mega connected to a MAX232 chip which is connected to a 56K faxmodem which connects to the phone line. I have no working sketch yet and to be honest don't really know where to even begin. Any help at all would be appreciated.