[Users] 401 Unauthorized

[Joao Pereira]

Hello to all
Im trying to register SIP clients (Cisco IP phone and Planet VIP )that 
are located behind an ADSL line ( and behind NAT), and the first time 
they register, they receive the OK from OpenSER, but when I reboot the 
phone, they cant connect again. They receive "401 Unauthorized" from 
OpenSER.
OpenSER isnt returning "401 Unauthorized" in any part of openser.cfg

Im not using firewall.
Can the problem be in the type of NAT?

Can someone help me?

Thanks
Joao Pereira