Problem with Dahua OUTDOOR intercom VTO VTH and Asterisk: Retransmission timeout reached on transmission

I 've a dahua intercom system composed by:

  • a Dahua VTO2000A outdoor unit ()
  • a Dahua VTH1510 indoor unit

The system function only if I use an old version of Asterisk with chan_sip (Asterisk 11) and only if the VTO calls a group call (one of the member of the group call is the VTH indoor unit)
Now I’ve made an automation AGI script that in some case (based on a DB query) instead of calling the Group Call answer the call from the outdoor UNIT and plays a ulaw audio file. The script is OK becouse i tested it with zoiper and other telephone. But when I used it with the VTO intercom the script start but after some seconds asterisk fail the call and hangup with the following errors:

[2020-12-16 15:48:16] WARNING [109134]: chan_sip.c : 4038 retrans_pkt : Retransmission timeout reached on transmission … for seqno 21 (Critical Response) – See [SIP Retransmissions - Asterisk Project - Asterisk Project
Packet timed out after 6400ms with no response
[2020-12-16 15:48:16] WARNING [109134]: chan_sip.c : 4067 retrans_pkt : Hanging up call … no reply to our critical packet (see [SIP ).

I attach two capture i made with tcp dump: one is with the right flow of the call the other one is when it fails.
My FreePBX ip is
The VTO ip is
The VTH ip is

Thank U for everyone who will help me in debugging and resolving this problem.
At this link U’ll find two capture file of the problem. One is about the correct behaviour the second one is when the retrasmission event occurs…and the call is hanged up.

Is Asterisk sending the response to the correct address? Is Asterisk sending a usable Contact address in the response?

By group call I assume you mean Dial with &'s in the dial string (this forum doesn’t support FreePBX, so FreePBX related issues need to be converted to underlying Asterisk ones.

yes it’s freepbx. but i don’t think that this is the problem. the VTO ip is asterisk IP is Why the script is OK with zoiper UA or with other UA except from the VTO?

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