Error message iphone 6 plus - Your SMS mailbox is full. New messages cannot be received until you delete some messages

I've had this issue 4-5 times in the last month. All my texts have been deleted. My latest backup has 6 total texts on it. After a couple days I receive this message and no longer receive SMS texts, but can send. Resetting settings allows one text only to come through.

Need to know how to fix this without restoring my phone every few days.

thanks!