W52P Lack of RTP activity

[mahatma]

Hi.

i have freepbx 13 running with asterisk 13.17.0, and a lot phones from diffrent manufactures, but most of them are W52P.
Phones are connected with chan_sip driver.

we have now a few strange errors.

When i make a call from any of the w52p, and i end the call, the other party stays connected, and i get an error in the log: chan_sip.c: Disconnecting call 'SIP/899-0000085f' for lack of RTP activity in 31 seconds

same happens when i try to put the caller on hold, or try to transfer them.
he will not get the MOH and after 30 seconds the call ends.

All other phones work fine, and i already double checked the network, all ports are open.

somebody have an idea?

config and pcap attached.

andi