it seems to point to an AMR problem… (hurray !)
I’m not familiar with siemens, but do you think there is any parameter that could cause such a weird behaviour?
I don’t think so. Therefore I would tend to assume that either 3GPP or Siemens did a mistake somewhere.
Or perhaps it is still a problem with DTX ? Can you try to disable UL DTX for AMR calls ? that might improve the situation…