Thank you for posting your query on Microsoft Q&A. If you are opting for password less signin, then
Phone/SMS is often used as a verification step and this falls outside of the regular MFA traffic hence won't be charged by Microsoft.
However if any cost is associated by you network operator for receiving these OTP, that would be applicable by your network provider and not by Microsoft.
Microsoft only has an additional billing for MFA Phone/SMS.
Please do let me know if there are any further queries.
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