I have/had a Facebook account with a phone number. The account was left unused for a few years. Recently, I used my E-mail to reset its password, unfortunately, since the account was abandoned for a long time, the system flagged my activity as "suspicious" automatically, and asks me for confirmation using my phone number. But I no longer own that number. And from the customer Q&A forum, I realized I was not the only…
I wonder if phone companies couldn't [partially] solve this; probably any system would be too open to abuse?