iPhone ringer

My friend has an iPhone 6s it will not ring when anyone calls or texts. If she notices it vibrating or is expecting a call there is no issue. The problem is simply she can’t hear it ring.

The software seems up to date, I’ve closed it down and restarted it on several occasions, in settings I’ve checked to make sure the ringer is at full volume and that the volume is not adjustable by the buttons.

Whats wrong and is there a fix ?

Thanks for any advice.

Kevin