Welcome, I am Cory.
Are you able to login to the account?
Please login to your account an go to the Account Security page here. - https://login.yahoo.com/account/security You should be able to change the phone number on the account.
Please let me know if this fixes the issue.
It probably wanted you to verify your login. When did you login last?
Can you get to the account details though?
Please click https://login.yahoo.com/account/challenge/username to try to reset the password.
Let me know if you need help with anything else