I'm surprised this has still not become a viable project. As it's an ideal system to swap data over very long distances. Probably due to issues/cost of getting a shield certified for connection to phone line.
Radicom seem to do some ideal looking modules
http://www.radi.com/modular51.htm
http://www.radi.com/modular29.htm
but probably not for hobby use.
Radicom specializes in designing and manufacturing modems for embedded system designers, original equipment manufacturers (OEM) and volume corporate users.
I just wish I had kept my old USR modems now.