There is limitation in Cross Tenant Sync: users that are SMS sign-in enabled will be skipped with error "Filter external users.alternativeSecurityIds EQUALS 'None'"
Ref: https://learn.microsoft.com/en-us/entra/identity/multi-tenant-organizations/cross-tenant-synchronization-configure?pivots=same-cloud-synchronization#symptom—users-are-skipped-because-sms-sign-in-is-enabled-on-the-user