In broadcasting they're usually referred to TBU's (Tele Balance Units) or Hybrid's. These units are usually very expensive but have auto-answer, echo cancellation, auto adjust for side-tone and the like.

If I was going to do it on the cheap I'd probably take an ordinary phone and hack the handset off. Then I'd grab a couple of isolating transformers and use them to bridge the phone to the conference system.

If you don't fancy hacking a phone apart, then a quick google turned up the Excalibur HC-1 (scroll down a bit), only $88. It doesn't auto answer though.