in your case depends actually what has been returned back in 200OK as reply to that posted PT 96  RE-INVITE from TEAMS. Maybe 200OK carries 101? by standard TEAMS should honor the answer


PT is not something you negotiate, offerer indicates what PT payload it will receive and would prefer to send however answerer may answer different PT and offerer has to comply sending different one, so it might be asymmetric.


If your "PSTN" reply also carries PT 96 back in 200OK that would be expected(but not mandatory) then MS misbehaves.


hope it helps.