Thanks for your question.
I've seen this a few times and it can usually be solved by reaching out to billing support. Billing support is completely free, but since you don't have a subscription yet you can dial the global service numbers here:
Also see the solutions here: https://learn.microsoft.com/en-us/answers/questions/2125640/i-am-unable-to-validate-my-phone-number-during-azu?comment=question#comment-1880744
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola