Phone number used on another account

That usually means the number is still tied to an old Apple ID somewhere. Easiest fix is signing in to your old account and removing or changing the number from the trusted phone section. If you can’t remember the login, try the account recovery page with your number and it’ll tell you which Apple ID it belongs to. After it’s cleared, you should be able to add it normally.

I question FirewallPulse's premise of not being able to use a phone number with another account. Some families may have younger members sharing a telephone number, or simply some who do not see the need for all individuals to have their own telephone. Until not all that long ago my wife and I had a single landline between us though we both had our own computers.

That seems pretty clear.... what is your question?