Error is "Verification code not sent. Something has gone wrong, please try again." I have tried on multiple occasions with multiple browsers. My mobile phone number already exists in Account Contact Information.
After literally months of fighting this issue, I finally resolved it largely on my own. The only Comcast employee that gets any credit is an asian male (name unknown) from the Lakewood CO service center who worked with me patiently and at least made numerous attempts to resolve the issue (unlick his female supervisor who repeatedly said "...different department..."). Even though he didn't resolve the issue, watching him interact with the system did give me some ideas of things to try.
The issue was that the mobile number I was trying to enter for two factory authentication was already assigned to one of my secondary users. Once deleted from the secondary user, I was able to add it as my mobile phone number for two factory authentication. If there was a more accurate & descriptive error message - such as "this mobile number is already in use in another account" - it would have saved me a tremendous amount of agrivation Comcast.