Hi @Johannes , I'm sorry you're having this issue. I answered a post similar to this as well a few weeks ago. The current solution for this is to use the Authenticator app or verification code instead of a phone number. If you absolutely need to use a phone number there are 2 steps for you to follow:
- Send me an email at "azcommunity@microsoft.com" with subject "ATTN: James Hamil" and your subscription ID.
- Contact the phone providers of the numbers having issues, and ask if they are flagged for some reason. Historically these issues have been caused by the phone providers, not Microsoft.
Please let me know if you have any questions and I can help you further.
If this answer helps you please mark "Accept Answer" so other users can reference it.
Thank you,
James