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Tap the "More" option on the Teams phone
Tap the "People" option from the menu

Tap the "+" icon in the bottom corner and tap "Add from Directory"

Search for the desired person/extension in the "Add" text box. (A keyboard will appear on the touch screen)
Tap the person's name.

Another menu will appear and select the "Speed Dial" option and then click the check mark in the top right Corner.

Now that person/extension will appear as an option when using the call function of your phone.