@riva thankyou for appreciating this idea from start Yeah that MOUSER module looks interesting ,However in case of CONEXANT I don't think that we need any kind of "approval" check the attached document this is a complete document for CONEXANT serial reference modem with all circuit diagrams,BOM and finished product which is available from CONEXANT which means they have already provided an approved product all we need to do is to take that circuit as it is and give it an Arduino shape thats it (I donot know its legal implications though 
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Just look into this document nothing is required to create from scratch
Link to CSM92 reference modem document
http://filesflash.com/19qv4mwc
reference modem daughter board and motherboard
Riva:
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.