Intermittent low volume on receiving side

I am experiencing a very low volume of the incoming side of the call.
It happens intermittently during the call, eg the volume starts normal, a couple of minutes hardly to hear and than suddenly the volume is at normal level.
Issue appears on several extensions. Also on a call with another FreeBX.
The configuration worked wel for five months until now.
FreePBX 2.11.0-19
Asterisk 11.17.1 R1
all extensions are VoiP Tiptel 3220.

Can anyone tell me in what area to look for an answer.