I only have one phone - my cell phone. This appears on My Account/Settings as my preferred phone number. I wanted to set up text alerts, so it wants me to add my mobile number. When I try to do so, it tells me that number already exists on my account.
Edit: I entered this under the Customer Service forum, but the system moved it here. This is on the Xfinity/ My Account webiste, not mobile.
TL;DR - Your cell number may still be tied to an old account, Comcast may have to merge the old account username with your existing one.
I recently had this same issue when trying to set up 2-step verification to sign in to my account. After calling comcast a bunch of times, we figured out that my number was still attached to an old account that I had with them. I had moved from state to state a few times, cancelled my previous accounts and then set up new accounts. Turns out that somehow, my cell phone was still attached to one of my old accounts, even though the account had been deactivated.
Their initial solution was to dissociate the number from that old account, but for some reason they could not do so or it just did not work (they said to wait 48 hours before attempting the 2-step verification set up, which I did).
Finally one of the billing and accounting folks (or mght have been someone from security department) who knew what they were doing suggested merging the Username from the previous account with the current one. They basically added the old username to my new (current) account as an authorized user. And that ensured the phone number was only tied to one account - the current one.
I was then able to complete the initial set up I wanted without getting a message that the number already existed.