Hey Jacques here from Support Engineering!

The 'email' and 'phone' fields are not inherently combined to serve as a unique identifier for Users in Intercom. However, it is common practice to use email as a unique identifier, and it is retained by default if one is present.

So the recommended way to uniquely identify users, would be to use the 'id', ‘email’ or 'external_id' provided by Intercom.