Iphone 6S has no volume on incoming calls!

Hi

Can anyone help, I have just noticed when receiving an incoming call on my iphone, there is no sound, the phone just vibrates.

I have checked the following -

1. Do not disturb - Switched off
2. Phone using 11.2.6 iOS
3. Volume set to maximum
4. Announce calls - Always
5. Music plays through speaker so draw the conclusion that the speaker is OK
6. No sound when receiving text messages
7. When changing ring tone, each one can be heard

Any ideas?