Cyberdata Intercom one way audio for 5 seconds

Hi,

I have a cyberdata v3 intercom with an issue. When the phone user answers to the intercom, there is one way audio : from the intercom to the phone works correctly, but from the phone to the intercom needs  5 seconds before start the audio channel.

The phone and the intercom are on the same network, so it can't be a nat problem.

I tried to change from PJSIP to SIP extensions without success.

I opened a case with cyberdata: I performed hard reset and firmware upgrade without resolve this issue. The configurations on the cyberdata are correct.

The issue seems to happen because asterisk is not able to learn quickly the intercom RTP destination :

Asterisk shows the first RTP Learning immediatly 0x368b8c0 – Strict RTP learning after remote address set to: 192.168.100.103:5004

But the second RTP Learning (the one  that points to the intercom) is shown only after 4- 5 seconds.

0x7f76e03e7130 – Strict RTP learning after remote address set to: 192.168.100.168:10500

When this log is displayed the audio channel is open.

The intercom has the RTP port fixed to 10500 on its configuration. Cyberdata said that this should not be a problem.

As the intercom is expensive, I'd like to do not change it if possible. 

Which could be the problem?

Thank  you