I asked that same question and was told that there were basically NAT issues but the ultimate solution was to use stun. It wa a weird probelm because it only was an issue when calling certain numbers and those numbers happened to be running GrandStreams and the issue only started when I switched from a GrandStream to a pap2t. It took them a while to figure it out so it would be interesting if someone could give some detailed info on what exactly it was and how they fixed it.

RA