W52P Non audio on some external calls

[dstamour]

We have a W52P with firmware v25.80.0.15. The problem we have is that some external calls have no audio both ways. The only way to get the audio is to put de call on hold and pull it back. I've done a TCPdump to get more details and here's what I've found:

(All this is from the phone perspective)
Sends SIP INVITE
Gets SIP 100 Trying
Gets SIP 183 Session Progress
(From there RTP in both ways)
Gets SIP 180 Ringing
(Here the phone send RTCP Goodbye and stop sending RTP and send ICMP Port unreachable when receving RTP from PBX)

So my question is, why does the W52P closes his RTP port when receiving the 180 Ringing at this point? The only way to fix this is to put the call on hold and pull it back so this will send an INVITE to the PBX and the RTP from the PBX will be accepted by the W52P.

Thanks