[iStudent 2003]

I am looking for a freeware program that will disconnect my dial up connection when a phone call comes in.

[Angus_D]

Google/VersionTracker for internet call waiting (+mac). I'd be surprised if you found a free solution (it'd probably cost you money every time someone calls you while you're online, because you have to use the call redirect service provided by your telephone company).

[OptimusG4]

Or you could have your mac pop up a window telling you when a call is in giving you the option to disconnect and answer or stay online (built in to the OS, check your modem settings).