A couple of years ago, we discovered that AT&T geoblocks all VOIP traffic from India. We sent some configured phones to them, because the bosses were told by AT&T that it would work fine. Nope. All the tracert results clearly showed them dropping the traffic. But then a few months ago, it started working! The India team emailed me asking to change out the names/extensions, and since then we've had no problems.
