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Hello Nhan Tran,
contact support via one of the contacts mentioned here https://support.microsoft.com/en-us/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
This issue with phone number validation during Azure Subscription Sign up can be resolved by creating a support ticket and working with our Azure subscription team.
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola