Device not ringing on incoming calls

[MamaG]

I frequently miss calls because my device doesn't ring.  I only know I've been called when I get an alert or see a missed call on the device

[Cleoriff]

@MamaG 

Just go into settings and check if your phone is on the highest sound call setting

[MI5]

@MamaG 

Check for any network issues here O2 Service Status 

If the issue persists, you may want to try a new sim card.

You'll need to call customer service.
Numbers here Guide: Coronavirus Community Help and Support 
When it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say "Upgrade", "Lost/Stolen" or "Fraud" if you have to, to get put through to someone.
8.00am is the best time to call if you can.