*Some* calls to my iPhone go straight to voice mail
I have been finding that some calls to my iPhone are going straight to voicemail. This can happen to calls from regular contacts or first time callers. However other calls just ring as normal.
The only 'fix' I have found is to turn Airplane Mode on and then off again. If I do this numbers that previously went to voicemail can get through for a time but then start going back to voicemail again in a few days later (possibly after I've turned the phone off and on).
I have done all the checks suggested in previous threads on calls going to voicemail. In particular, I have made sure that "Silence Unknown Callers" is off and that none of the calls are coming from the three numbers in Blocked Contacts.
Any other suggestions?
iPhone 4s, iOS 9