Phone Number Associated With My Old Account

I was recently trying to verify my phone number from my Xfinity account.  Upon attempting to do so, I received the following message: "This phone number is already associated with an Xfinity account." I didn't check but I suspect that my personal e-mail would have the same issue.  What steps might I need to take to to unlink this data from my old Xfinity account (different user name as well as inactive?

Thank you