Receiving message that "This phone number is already associated with an Xfinity account" when it is

I am receiving a message that "Receiving message that "This phone number is already associated with an Xfinity account" when I try to establish two-step verification on my account.  However, I know the phone in question is not currently in use on another account, as it is my number.  I was at one time used on another account, but that account has been deleted.  How can I get Xfinity to recognize this is the case?