Azure AD Premium and its capabilities is not currently available in Tenants hosted in our Mainland China Azure AD instance such as when a company signs up for Office 365 or Azure operated by our partner 21Vianet.The SMS will works and there are other reset option
A company with Tenant in our Global Azure AD instance, hosted in our global datacenters, has access to Azure AD Premium services and all employees in that Tenant, including those in China, can leverage the services.
We do have other options available for both SSPR and MFA that do not require SMS/Phone calls (e.g. Azure Authenticator app for MFA and Q/A gate for SSPR) but does require internet connectivity.https://techcommunity.microsoft.com/t5/Enterprise-Mobility-Security/AzureAD-Mailbag-International-Deployments-Round-2/ba-p/250025
Fun Fact: For Azure MFA, you can change the Caller ID Phone Number but this is only from US phone numbers only.
Azure AD Password Reset is localized into the full Office language set. SSPR uses the user's browser locale when choosing how to localize the SSPR page and all of its communication (SMS, voice, question languages).https://techcommunity.microsoft.com/t5/Enterprise-Mobility-Security/AzureAD-Mailbag-International-Deployments/ba-p/250020
That will work if number is valid one