Users can't change their office phone number when they set up Azure Multi-Factor Authentication
Office 365 ProPlus is being renamed to Microsoft 365 Apps for enterprise. For more information about this change, read this blog post.
When a user tries to set up Azure Multi-Factor Authentication, the user can't select or change the Office Phone option. The user can't edit the phone number because the text box is unavailable (appears dimmed). Additionally, the user can't set the office phone as the preferred contact method.
This problem occurs for on-premises users who are synchronized from the on-premises Active Directory environment to Azure Active Directory.
Change the user's office phone number in the on-premises Active Directory.
For more information about Azure Multi-Factor Authentication, see Azure Multi-Factor Authentication.