Poor audibility of ip-telephony - what is the reason?

Question

Telephony

We call through ip-telephony. The provider is sipmarket.net, or rather it is not a provider but an "aggregator" of providers.
We use a softphone for calls. A bunch of simple softphone -> sipmarket.net -> exit to the city
There are problems with the sound. Gurgling, quietly audible to me and them. And all this is unstable, sometimes good, sometimes bad.
How to figure out where the cause of a bad connection? What parameters to measure ping, use another provider? What are the tips? Where to begin?

Answer 1

We call to Moscow on city or cellular.

Answer 2

Now we are connecting another provider, we will try to call through it. Maybe it's our internet. Through sipnet - also tried gurgling.

Answer 3

It turned out to be in the sip client. MicroSip gurgles, PortGo doesn't. There is no exact clarity.