Yes, but as said above another SIP client (linphone one) has the same problem.
Yes, assumption and analysis correct.
But logging all RTP would be against the law to ask for permission before recording calls, or? And it would make use of quite extensive disk space?
After some more testing in bed space and this morning it seems related to my operator blocking audio packet on cellular data.
With another SIM in the Android phone it works perfectly via cellular data.
So I may test that when I have opportunity to get to another WLAN (not behind a FRITZ!), which can be delayed, virus caused.