Hi Fez6911, welcome to the forums.
This issue has been brought up to the team and has been under investigation, however we found attempting adding a phone number and the authentication could label that number as suspicious as a security measure. There are various reasons why adding a phone number can fail, one of which is simply entering the country code incorrectly or the number itself.
Waiting till a later date to add the phone number to the account can usually resolve the issue and please make sure to avoid numerous repeat authentication requests, as this can also cause your phone number to be labelled as suspicious preventing you from adding it to the account at that time.
If you continue to experience a problem adding your phone number to the account, more details about our SMS service can be viewed on this support article, here -
https://support.ubisoft.com/Faqs/000054420