Audio problems Voxalot --> Betamax softphone
 

I am using Voxalot so I can phone with my Siemens Gigaset C470 IP to a Betamax softphone (Poivy).

In the Siemens I put the Voxalot settings, and in Voxalot I set up speed dial.
When I phone **1 on the Siemens I am getting a connection with a Poivy user that is only having a softphone. In Voxalot I set up Poivy as my Voip provider as well.

This worked OK for almost a year, but the last few months the audio is very bad. The other side can hear me very well, but on my side there is a lot of hum. It was not like this before, this is from the last few months.
I did not make any changes in the Siemens nor did I change my Voxalot settings.

Any suggestions how to get rid of this hum?