SIP registration, 1 phone registers and "seizes" 5

Originally we thought it was an aastra problem, but we have switched to polycom and the problem still haunts us.

  • is behind NAT (we moved it out from behind the firewall for troubleshooting but no dice!)

SIP phones are also behind NAT

The issue is that the first phone registers and practically ‘seizes’ port 5060 which does not allow any other phone to undergo sip registration. The first phone to register, wins! The rest of the phones get hit with a 401 unauthorized over and over.

We have tested aastra as well as polycom, we have moved * from behind the firewall, and we have had absolutely no luck.

Global settings: NAT = YES, externip = XXXXXXXXX, localnet =

User settings: NAT = YES, Qualify = YES

We know each phone is configured properly because we can get any phone we want to register perfectly, just one at a time. Does anyone have any ideas on this single user registering behind nat, seizing 5060, and producing a 401 unauthorized for every other sip phone that tries to register from that network?