As per Microsoft Faq.
https://azure.microsoft.com/en-us/free/free-account-faq/
We use the phone number and credit card or debit card for identity verification to validate that account holders are real people and not bots. We don't charge your credit card or debit card anything when you sign up for Azure, but you may see a one-dollar verification hold on your credit card or debit card account. The hold is temporary and will be removed, never charged to you.
Also check this thread.
https://learn.microsoft.com/en-us/answers/questions/433827/how-to-get-an-azure-account-without-credit-card.html