[OpenSIPS-Users] To-tag value in ACK

John Quick john.quick at smartvox.co.uk
Wed Feb 17 14:57:24 EST 2021

I took a fresh look at this case and found that the To-tag used in the ACK matches the To-tag that was in the first 180 Ringing.
However, Teams has returned different To-tags in the 180 Ringing and in the 183 Session Progress and 200 OK.

This appears to be common behaviour for Teams as I can see the same thing in the SIP trace for a completely different customer.

Teams uses different To-tags when it responds to an INVITE as follows:
SBC             <------>          Teams
INVITE   -->
<--  100 Trying        no To-tag
<--  180 Ringing        To-tag A
<--  180 Ringing        To-tag B
<--  180 Ringing        To-tag C
<--  183 Sess.Prog.   To-tag D
<--  200 OK               To-tag D

Is this compliant with the RFC's?

Unfortunately, the UAC used by my customer is responding to the 200 OK with an ACK that has To-tag A instead of To-tag D.

John Quick
Smartvox Limited

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