Check the integration log
If the integration log shows "Errors: Phone number has already been taken" (as per image below)
It means that, in Cloud Assess this user exists in another organisation using a different email, with that phone number
Suggestion/Workaround:
Edit the student in PowerPro
Clear the mobile number in PowerPro (save it to the PP Client notes perhaps)
Once the student "lands" in Cloud Assess (should happen shortly), edit the student in PowerPro and re-enter the mobile number