it is indeed complex.. the codec used can’t affect the roundtrip time. The time of travel is still the same, whether you are in FR or HR.
Maybe your issue is more related to the fact that the “AMR change orders” arrives too late ?
For a while, just to test : Can you try to “freeze” your AMR codecs to only one codec in HR (a safe one) and one codec in FR (a safe one as well).
You might degrade voice quality a little bit, but at least you can isolate your problem:
1) if no more call drops : it is linked to the process of changing the codecs during the call.
2) if call drops are still here :
2a) disable AMR FR
2b) enable AMR FR and disable AMR HR
2c) disable all AMR and use only EFR
that should give you an idea of where the problem is occuring 🙂