TG800 On GSM side voice is disrupted on random calls

While using TG800 on about 3-5% of calls we encounter problem with voice on one side.
Computer <---[sip / udp voip]---> TG800 <---[gsm] ---> GSM Phone

We have recorded audio on both ends.
Below two calls, each with two recordings (one per VOIP side, one per GSM side):
call1 - voip_side: https://ufile.io/oyegrlxx
call1 - gsm_side: https://ufile.io/och2bqgu
call2 - voip_side: https://ufile.io/7lx6dg2n
call2 - gsm_side: https://ufile.io/bmahyvdj

Record on VOIP side sounds good. Recording is made by sniffing udp traffic between A and TG800, so this is exacly what goes into TG800. So our voip softphone works fine.
Record on GSM side is bad - voice of voip caller is hardly recognizable.

We had TG200 on tests before in the same spot and problem didn't occured. We also have some GSM phones in the same location with the same GSM provider and problem doesn't occur. So it's not a problem of signal strength etc. Espessialy that voip caller hears gsm caller fine.

Looks like TG800 sometimes is messing with sound conversion from VOIP to GSM. We have tried to change all settings - echo cancelation, volume, etc. nothing helps.

Anyone has encountered simmilar problem? Any ideas?


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