My phone wasn't working properly. I wasn't getting phone calls - or rather it was telling me afterwards that I'd missed one.
I checked the settings and everything seemed to be okay so I was bemused. I decided to try Vodaphone Help.
'Have you checked the Do Not Disturb setting?'
'No, of course not. I wouldn't have it set on Do Not ... oh, well, that would explain it then.'
I didn't even know there was a Do Not Disturb setting so how it got set is a mystery.