How to stop Google using my old iPhone for verification?

Every now and then I have to re-sign into Gmail etc. That's fine. However, every time I do this, it asks me to use my OLD iPhone for verification. How do I stop it doing this?

I have checked the list of devices at https://myaccount.google.com/device-activity and my old phone is not there. My current iPhone and Macbook Air are both there. I can't remove a device that isn't there.

I still have access to my old phone. I signed it out of iCloud and wiped it and gave it & the SIM card to a family member. They now have it signed into their own iCloud account and their own gmail account. Maybe Google thinks the number is still mine? A month ago I checked all phone numbers linked to my account, and updated my number and the 2-step verification numbers to my current phone. That was a month ago & I'm still having this problem.

I've just checked with the family member - their Gmail 2-step authentication is indeed linked to their own number (the one that used to be mine). Their number shouldn't be coming up as the number for my own 2-step verification.

Any help?