6. Adding Outside Contacts

On the Calls screen there is a Contacts section. To add an outside contact click on the Add Contact button.
Teams will then prompt you to enter a name or number.
If you add an outside number and push enter, it will prompt you to fill in the rest of the information for the contact. It will fill in the Full Name with the number by default, so be sure to change it.
You can also enter a name, but if their name matches someone in the organization you will get them instead.