The NAT function on the US servers began to have issues long time ago. The issue was reported
here but did not receive much attention. Most phones with STUN properly set up won't notice the problem and some phones works even without STUN.
One thing you can try is to make sure STUN works properly on your phone though it may not solve the problem. Use only UDP protocol. TCP seems to have more trouble. If your phone still does not work, you have to choose EU or AU servers.