My E3 is currently sitting running KPhone/Pi under OPIE. Annoyingly I currently have to have a USB mouse attached due to lack of a suitable pointing device on the E3, but I'm sure that's solvable in time.
More disappointing is the fact that it doesn't really work. :( I can successfully SIP register with my local Asterisk instance, and dial voicemail successfully. I only hear a fragment of the "Chameleon mail" prompt and attempting to send DTMF to choose a mailbox doesn't work either - I don't know if this is because I leave it too late or if they don't get sent at all.
OpenEmbedded doesn't have any other OPIE based SIP or IAX clients, nor anything text mode that I can see. I found ZiaxPhone but it seems to be closed source so I can't rebuild it. Guess I'll have to find a text mode client and try to work out if it's KPhone, the audio driver or the E3 not being fast enough that's the problem.