GSM codec bug ? very strange noises

Hi there,
fresh new asterisk 15.3.0 install on debian 9: everything is fine except GSM codec not working: I send and receive no sound.
every other codec works fine (same context, same parameters)
I use chan_sip
no NAT involved (user AND server)
using x-lite, sometimes I can hear some very strange “metalic” sounds
thanks for your support.

OK, I managed to record the sounds:
strange noises

those are the sounds coming from an echo test, using GSM codec. With any other codec, the sound is excellent.
On some clients, I will hear no sound at all (the client codec must consider that the data are errors and therefore produces no sound?)
it’s not a nat or firewall problem: the RTP data is flowing correctly.

this happens with the GSM codec when I compile asterisk myself, and also with already compiled versions available in the debian depots.
I use a standart VPS. The problem arises since I went from debian 8 to debian 9 (not sure of the causal link, other parameters have changed).

workarround woule be to disable gsm codec, but I would like to know what’s going on here…

Problem has not been solved…