what about this Board?http://www.e-lins.com/EN/index.html
Dial Up modem's days are long gone but Land line still exists either in the form of home pbx or PSTN, An Arduino based Dial Up modem shield can be used for a lot of fun projects. I'm no electronics expert but modem chips are already available with AT Command support http://www.conexant.com/Product/modems/embmodems/CX93040/Pages/default.aspx I think Arduino can produce this shield in no time what do you think? :~
Radicom specializes in designing and manufacturing modems for embedded system designers, original equipment manufacturers (OEM) and volume corporate users.
I was also investigating the use of warning module arduino that can call another phone without it being rung up and using the ring tone and length of call to issue basic information to another module that listens for ringing and length of ringing .