Question: AMR 2G problem

[josef]

Dear brothers,

We have a problem of changing codecs during AMR calls: when the call is started it stay on the initial codec (5.9 kbps) with good radio conditions. But sometimes it is doing upgrades (in the same radio conditions): random behaviour

The problem is not related to radio parameters: we checked all the possible values

The problem is not related to the implementation: we checked implementation PCM by PCM

The problem is not related to licenses: we checked licences for AMR FR and AMR HR and evrything is OK

To what can be related this problem?

Thank you brothers

Josef.