Sipx + Cisco 7960 + Nat

Hi,

I'm using sipfoundry using the vmware machine and I have a small nat problem.

I guess there would be some option in the configuration to force the nat when using cisco phone but i cannot find it.

My design :

(cisco 7960)-(nat router)-(internet)-(sipx server)

My diagnostic is the following :

when i have someone else trying to call me, my phone rings, but there is no voice or other signal when i take the phone.

When i phone someone else, he get the ring, answer and communication is established fine.

Note that the other user is using a softphone also behind a nat router.

Any clue ?

Regards.