1. Can you traceroute from the mikrotik running as the pppoe server, to the internet?
2. Have you got the users getting public routable addresses, or are you Natting them from an IP you give them off the mikrotik to a routable address?

There is definately an issue with the Mikrotik maintaining and tracking a SIP connection. Obviously becomes more of an issue with multiple NAT's. For some reason, different SIP kit seems to manage/handle this better, but we have found that specifically with Quintum kit, everything breaks. This was m...