If I may ask a silly question - which bit is actually connected to the local telephone exchange? I would guess it's an ADSL router.
The reason I ask is, some part of the hardware has to be licensed for connection by voice. On a PC, they typically have a built in modem heavily backed up by licensed drivers.
On a PI, you don't have that. The system wasn't designed for connection to the public telephone network, and ADSL routers aren't licensed for carrying analog voice data over phone lines (you need a telephone plugged in to do that). Modems give analog output and read analog input.
Anyway you could plug a landline phone in alongside and just listen for differences in the exchange? That might highlight the problem, and is a very useful way of monitoring differences between "real" modem connections.
It's a lot easier when dealing with mobile phone systems, because they are completely digital. Not that I do that sort of thing. Although I have seen other people doing similar things.
Couple links that might help you - first is issue description, second is a "hand on" tutorial.
http://www.tldp.org/HOWTO/Modem-HOWTO-2.htmlhttp://extradio.sourceforge.net/extmodem.html
