We're not using them, but check out CallInStudio. For a self-funded Internet broadcast of any length, it could get costly, but for a successfully monetized show, the cost is reasonable.

The best and most expensive solution is to add some telephone lines and get a multi-line Telos digital hybrid system.

It's also possible to use an unlisted call on TalkShoe (free unless you get a direct dial number) or BlogTalkRadio (monthly fee, low quality) as a telephone bridge - NOT as a show hosting medium.