Why might it break the connection?

This:
Provider Rostelecom SIP
PBX Yeastar S20
Phones Cisco SPA504G

ACC registered provider, the phone is registered to the PBX.
When you call that incoming that outgoing, the connection is torn through 186 seconds.

Searched through the settings of the PBX, there is no clear guidance for such timeouts.
Scheduled for tomorrow the relationship with TP RT, together we will watch the log SIPA.

Question to experts, perhaps it is time you have a idea which way to look.
Perhaps at some proprose breaks the connection, but details will tell only of the log.
No one is silent when speaking.
June 10th 19 at 15:30
3 answers
June 10th 19 at 15:32
You have Wireshark forbidden to use?
The output Vari shark not every admin informative, it was not just the provider 186ой second makes a INFO request, no response hangs up. - Henry32 commented on June 10th 19 at 15:35
June 10th 19 at 15:34
Go to S20 via SSH, there run 2 instance of tcpdump on the external interface to the operator's side and on the inner side of the phones. Something like: tcpdump -i eth0 -n-s0 -w /tmp/dump-eth0.pcap
It is advisable to run asterisk-rvvvvv to collect data from the console. Just in case.

Next, make a test call, waiting for the "cliff" after 186 seconds. Interrupt tcpdump, download on desktop both dump open both above-mentioned Wireshark-ohms, look what happened (menu Telephony - Sip flows much help).

Question time - 186 seconds too much, not to comply with any timings SIP and interrupt the conversation. Usually 15 or 30 seconds - lost "ACK". A 186 - probably somewhere mistakenly billed duration of the call. Can be NAT loses "the stream", but then was interrupted by the voice (RTP) and the call would continue.
June 10th 19 at 15:36
Noise reduction turn on the phone

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