Yealink WH63 - Fading voice

[grb33]

Hello,

We have 15 sets of handset WH63 and phones T42S.

It often happens to some users that when they speak, their voice suddenly moves away for 2-3 seconds and then comes back again (It's as if they leave the handset and then come back again.)

I have reset the phone and handset to factory settings many times and updated the firmware to the latest version. Unfortunately, the problem randomly returns from time to time.

Where could be the cause of this problem?