Had this problem. Went to my carrier and they said it’s very common. Went to Apple and they immediately acknowledged that it was a glitch and a non fixable problem that is specific to the iPhone 7.
If you have Apple Care, they give you a free new phone right then and there.
If you do not have apple care, then they send your phone in to get fixed and give you loaner phone in the meantime. They said it takes about 5 days to get your original phone back.