[asterisk-users] Grandstream

[Phibee Network Operation Center]

I have a problem connecting a Grandstream ipphone to an asterisk.

The ipphone is behind a nat router, I redirected UDP 5060 and 5004 to my
phone.
It connects well to the asterisk server. I can call outside and receive
calls from outside without any problems.

But if I call from this ipphone to another ipphone connected on the same
asterisk server, using internal dialing, I can hear my correspondant, but he
cannot.

Do you have any idea?
Thanks for advance.