Problem with Siemens "Poppy" phone not ringing.?

I can set the ring volume and it makes the right sample noises.

When however someone calls the number, the front screen illuminates; but it doesn't make any ringing noise. If you answer the 'phone you can talk with the other person. Nor will it make any noises when a text arrives, although it has been set to do that too.

It's been turned off & on again, Battery has been taken out and replaced, and we have tried an alternative SIM. It was originally with Virgin, but has been unlocked to all networks.

Any constructive suggestions out there?