Well, as long as you are able to convert a regular landline to voip it should be possible.
Try to get something like a linksys pap2t or similar and connect it to an ethernet port of your router.
Then install asterisk and some tools from here:
http://ipkg.nslu2-linux.org/feeds/op.../cross/stable/
download from the internet: The future of telephony, Asterisk
Read the book and configure your telephone server
Here you'll find an excellent starting point:
http://www.voip-info.org/wiki/
And here you find the possibilities of asterisk to record directly a phone call.
Consider, that recording a phone-call is forbidden by law.
http://www.voip-info.org/wiki/view/A...ation+commands
Section: "Sounds: Recording and monitoring (listening-in)"
The big advantage is, that you can record any phone call handled by asterisk. A Wl500gP can handle up to 4-5 phone calls simultaneously, depending to the other services handled by router.
And a last hint: At ebay you can get sometimes a fritzbox-fon from AVM pretty cheap (mostly for 9-10 Euro) or other cheap equipment for voip.
Maybe your concept will work too, but it is very hard to get all things together and running.
In any case it needs a lot of time and configurations. I bet, you'll find thousands of config-examples for asterisk and it is easy possible to put everything together.
In case you start the project, be so kind and share your experience with us.....I bet many forum users are able to help you a bit.....
have fun