Hi Lermontov, welcome to the community.
I appreciate your frustration, but it would be best if your assistant had continued working with our support representative. I've read through the support case notes and the rep's conversation with our L2 support while they were talking with your assistant, and this appears to be a technical issue and not an issue with the SMS Registration.
Do you have another iPhone or an Android phone that you could install our App on and sign into your account with as a test to see if the same issues happen?