The telephone rings but no voice is heared

Hello, everyone.
I’m using asterisk in a cable network (EMTAs that support SIP). Here is my problem:
A client is making a call, on the other end the telefone is ringing. When the person picks up the phone, he doesn’t hear anything. The person who have made a call hears “busy tone”. In the asterisk logs I see this call like a normal “ANSWERED” call. The duration of the call is from 0 to 5 seconds. No errors. I’m starting to think its an issue of cable modems, but anyway would like to ask if anyone had experienced something like this in the past.
Thanks in advance.

Hello, friend:
I think if you are in LAN, there should not be a problem. if one of you is not in LAN, it will be a problem because of NAT. i think you have to configre the NAT first and test agian. good luck

Yes, the problem occurs inside the LAN (cable network). I have nat disabled.

post a log file fragment for a failed call, and the section from extensions.conf that this call uses.