Here is what I had tested so far:
- Since a PAP2v2 has a support for a SIP Registrar Server which is identified on its Registrar Server field, I put it to some tests and configured it with my old VoIPBuster account using the information you provided.
- Regardless if I configured my PAP2v2 with a sip.voipbusterpro.com string on its Registrar Server field or not, I was able to call/converse with my UK/US numbers.
So, it looks like this isn't an issue at all on my tests, AFAIC. If your E51 can support STUN options, try to use it to see if this will resolve the issue.