How does VoLTE work? Why doesn't 4G+ work?

Question

VoIP

MegaFon recently brought VoLTE to our city. However, everything is fine only within the network. When calling from a smartphone with a MegaFon SIM card with VoLTE support to another MegaFon subscriber with VoLTE support, then

HD challenge

When calling a MegaFon subscriber to a push-button phone without VoLTE, the call goes through 4G, but

not in HD

When calling a Beeline subscriber with VoLTE, the call goes through 4G

in standard quality

But if you call a Beeline subscriber without VoLTE, the phone gives an error

server is not available:

How does SIP VoIP VoLTE work in general? How to make calls and SMS / MMS work normally with all operators and subscribers?

Answer 1

SIP VoIP VoLTE is a more advanced calling system.
It is not in your power to make calls and SMS / MMS work normally with all operators and subscribers. But you can call your network operator, he can help you.
Good luck! I want to solve your problem.