MagiCall works wonders for stuff like that. not sure who makes it...give me a few minutes and I'll look at mine here.
edit: MagiCall by mobiion- info can be found at the following link:
http://www.mobiion.com/magicall.html
you can download a copy here if you want- install is pretty self-explanatory:
MagiCall.zip
I know a few others that have used the CallFirewall that tmz recommended, but I have no experience with it myself.