All I have is a simple phone that doesn't have very many features, so I'm no expert on these (or even other) things. However, I know when Mrs TW's phone gave trouble, we took the battery out of it for a while and then put it back in again, This sorted it out. If the cause of the problem isn't just a setting somewhere, perhaps that could be tried.
I know that phones can be reset and this too might cure the problem but you have to be careful not to wipe everything out in the process. Maybe the safest thing to do would be to take the phone into a phone shop for advice.
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