Do you use Whatsapp? Unless you opted out during the short window they offered during the summer, Facebook got all your Whatsapp contacts and data, and that of course includes your phone number:
https://www.cnet.com/how-to/how-to-stop-whatsapp-from-sharin...
It'd be very easy for Facebook to cross-reference multiple address books (even 1 would be enough) and create "virtual profiles". E.g. if Alice is not part of the Facebook/Whatsapp ecosystem but Bob, Charlie and Donna are, and they all have Alice's phone number (unique identifier!) on their phone. If all 4 hang out together a lot and Bob, Charlie and Donna upload a lot of pictures of all of them, and they diligently tag people who have accounts, then Facebook can probably figure out the 4th person (which face they've stored for their recognition engine) is probably Alice.
And if Alice, like the poster, never gave Facebook her phone number, but Bob, Charlie and Donna have it under "Alice Lastname", and they're all friends with Alice Lastname on Facebook, Facebook can probably be certain to say "Store your phone number, we think this is it.". Or to word that properly, "Confirm your number, although we already know this is it."
