Good luck, you’re going to need it!
I had the same with 4 Cadets out of 15 registered onto SMS the same night affected. It took over six months of ‘discussions’ with the Bader Team at HQAC before it got resolved. I say discussions as they kept telling me if was a problem our end, clear cookies, empty caches, use a different computer…you name it, I was told to do it. They then did something which left me with 2 Cadets still having the same issue, and we went back round the houses even with me telling them to just do whatever they did to the other two user accounts.
I was on the verge of deleting them and adding them as new Cadets on SMS when the Wg Cdr stepped in and phoned the Bader Helpdesk every other day on my behalf until it was resolved. To this day I don’t know what the problem was and how they fixed it - sorry!