I have the same problem with my @outlook.com email address with phone number +84
The issue caused me a terrible situation when my account was locked because I started importing my data from Google Cloud (REQ No. 7060849998).
I cannot use my phone number or any else with area code +84. Until now, I have not been able to add my phone number to my account for verification method.
The staff who supported me on that ticket could not provide any other solution excepting I must enter a foreign phone number to get a code to unlock my account while my other verification methods such as recovery email address and Authenticator application were not allowed.
This policy from Microsoft has made a critical convenience to us. Can some expert staff review it?