Phone number is already associated with an Xfinity account

A secondary user on my account notice that the phone number listed for them is NOT their phone number (and I don't know this number either).  I would like to change it to her number but keeping getting this error:  This phone number is already associated with an Xfinity account.

I checked all the secondary users and this number is not used for any of them.  I need to update it to either her number or mine, but my number is already associated to my email so that isn't allowed either.  

I did remove the number that is not known to us so whoever that was can't access the account.

How can I fix this?