From your Sharepoint and MFA installed server,
Step 1: Check the URL of Sharepoint site to secure with Azure Multi-Factor Authentication
Step 2: Add Multi-Factor Authentication for HTTP websites with following base URL of Sharepoint site
Step 3: Check users added in "Multi-Factor Authentication server"
From an external system,
Step 1: Browse URL of Sharepoint Site Enter a user credential to login - must be a user in MFA
Step 2: Enter a user credential to login - must be a user in MFA
=== Authentication phone rings ===