Try contacting your ISP and check to see if any voice ports are being blocked. Sometimes ISP's block certain ports to protect against viruses/trojans/worms etc. It's a cheap, inexpensive way to protect a network, but it (obviously) creates bad customer service feedback. The usual response of an ISP when asked "Do you have my voice ports blocked?" Is "Sir, we usually don't block ports." Keep hassling your ISP and they will break. Ask if you can have a direct port connection to the yahoo/aol/msn voice server (whichever you use) and they should comply. This being, of course, if port blocking is the problem.
Ruse Of Metacarpi
Without me, this yin and yang would be one y short of a philosophy. ~Ruse of Metacarpi~