seperate phone lines both with call waiting
might be cheaper and faster to go to dsl or cable but i understand its not available everywhere.
But anyway here are a couple you can search the net for "Internet Call waiting" to find more
I've used Call Wave quite awhile ago it will popup a message saying you have a call and you have the choice to take the call or not. (also if you have caller ID it would show the caller)
Pagoo Didn't like this one but i'm sure they've updated since i used it.
Pagoo
http://www.pagoo.com/
Call wave
http://www.callwave.com/landing/CORP_Main.asp