Phone electronics and interfacing is a frequent topic on here, and I thought I would mention that I saw my old friend "Understanding Telephone Electronics" back in print at Borders.

It used to be thin, red, and $5 at Radio Shack.. Now it's thick, Green, and IIRC, about $30.  Still, a very good treatment, and if you're going to do anything on a POTS line, a good investment.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.