Different Local SIP port for each SIP account on W60P

[msncookie]

I believe I'm having a similar issue on the W60B where enabling a second account causes the registrations for both accounts to start flapping up and down. I am behind a firewall and the server is a VOIP provider across the country. If I disable one of the accounts, the remaining one works fine.

I believe the issue might be the single source port being shared across accounts. Is there anything I can try to help make this work behind a firewall? If source port is always the same for all accounts, how does the W60B direct return traffic to the correct account? Does it use SIP headers for this?

Thanks very much.