DHCP very bad. That is one thing I have noticed with many firewalls that claim to be SIP aware. The issue arrises when it transitions from public to private it does not keep the same port numbers and I would highly suggest to use Ingates line of products for SIP deployment. If you want to send me your SIP Peer Profile I might be able to make some suggestions on what to try but more then likely it is a known issue in most firewalls. NAT is a SIP killer.