"Phone number is already associated with an Xfinity account"

I have one Xfinity account and have had only that account for decades. Within that account, I have several e-mail address accounts. Some of those e-mail accounts show a phone number that I no longer use. When I try to change the number to my cell phone, I get the message "Phone number is already associated with an Xfinity account." I only have one phone number. Why can't all my e-mail addresses use the same phone number?