Next to the navigation you can add a call to action button that can be linked to any page internal or external.

The button can be edited under Appearance → Cusomtize → Header under the “Call to action button” settings.

If you’d like to link it to your phone number, just add this in the “Button link” field:

tel:123456789

Replace the number with your actual phone number, and make sure to remove any spaces or symbols from the number.