Fasting for Megafon subscribers.
The bottom line: the internet is lost during the call
If someone else ran into a problem when 4g network for an incoming / outgoing call switches on the internet is not 3g, and disappears entirely - there is a solution, but the steps below will have to play each time the phone is rebooted, or seizures Sims.
What to do: open the phone and enter * # * # 4636 # * # * then go to the SIM card option and change preferred network type as an attachment.
VoLTE to set up and failed, I will be grateful if someone will share the method.