[SR-Users] Problems with topos and UPDATE messages

[Simone de Blasiis]

Hello to everyone,
we are experiencing some issues with UPDATE messages being sent by the PSTN
after an early media announcement.
To reproduce the issue I have to call a specific number that has an early
media announcement, since it's a paid phone number it has to announce how
much it would cost before picking up the call. In this initial part the
call is being negotiated in g711a.
As soon as the announcement ends, the PSTN sends an UPDATE message
containing the new desidered codec from the callee which is g729.
At this moment Kamailio doesn't route the UPDATE message to the destination
and it actually replies with a 404 message.
I've attached a pcap with anonymized IPs to make it harder for bots.
Here is a legend for the IPs:

   -
   - 12.34.123.123 is Kamailio
   - 62.23.19.74 is the PSTN
   - 1.2.130.40 is the client making the call

I found a similar report <https://github.com/kamailio/kamailio/issues/2659>
in the GitHub issues of the repository and we tried forcing the
record_route() as suggested but, as the OP reported in his last message,
it's not working.
As soon as we disable topos, the UPDATE message is correctly relayed to the
caller.
In the documentation we can't find anything that could give us a hint on
what's happening.
Does anyone have any idea on what could be happening?
Thanks in advance

Simone
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