There are posts for this already, but i am happy i found this. here we go:
You have no ideia how many times i was in a situation that i thought about something like this! A fake call program, that you phone will "pretend" that is ringing so you can say to you boring neighbor, or that annoying metting: Oh, my God... sorry i gotta take this international call. excuse-me...
http://freewareppc.com/communication/fakecall.shtml