Question Author
Thanks all.
No, the default is 15 seconds and (with Virgin) it cannot be changed for those customers who joined after December 2017.
I've tried all your suggestions and alas none works. But thanks for all your efforts anyway. I'm going to try a factory default reset (in case I did anything when I was setting up ringtones, etc). Then I'm going to contact Virgin to see if they can help. If they both fail I'm going to consider whether to keep the phone or not. I'm in two minds because on one hand I think it is ridiculous to sell a phone which is so difficult to simply answer a call. On the other, with just 15 seconds before it switches to voicemail, it is most unlikely that I will get to answer most calls even if I could (since I don't walk about with a phone clutched in my hand) so I will have to ring the caller back anyway. I'll consider putting a voicemail message to that effect on my number. I can only laugh (with despair) at a company which makes a phone which you cannot answer!