Hi skeown,
Welcome to Centrify Community!
May I know which country code and the phone format you entered for this user? The call will be failed if this is an incorrect phone number or in an inappropriate format. Please make sure the phone number of the user is correct. Besides, stating the country code will make things clear. Here is the example, if the mobile phone is in Japan, the mobile number should be in this format: +81 11112222
If it is still failed, please add 'email confirmation code' or other challenges in Authentication Profiles to unblock user enrollment.
Hope this will help. Please let us know if you have further questions. Thank you.
Kind Regards,
Yeny