Hey I have the same problem, try if it goes away like it did for me if you turn off the loudspeaker when calling and then immediately switch back to the phone. If so please give me feedback.
My phone has already been sent in twice and they can't find the error, it must be a software problem. Support doesn't have a solution yet either, but they don't know what the problem is. Now I am glad to hear that from you. Give this to the support

Which network do you use that WifiCalling works?

Do not put your hand on the microphone. It is better to hold a cupped hand in front of the microphone.