No sound when making calls with iPhone 6

I'm using iPhone 6. Sometimes when I answer a phone call, there's no sound. Only when I answer again or call back can I hear the voice. Does anyone else have the same problem? Do you know the reason?

This is not so common but it has happened to all of us. It's usually due to a glitch in the cellular communication which can happen at any signal strength. It's of course possible that the person on the other end of the phone hit their mute button too. In any case if you are not able to reliably repeat the problem then it is due to some random occurrences in the cellular network and not anything within your control.