Once you have set up the phone capability in Archdesk, you can call form the system. If the SMS capability is also set up, the system will allow you to send messages as well.
To call from the system, go to the Contacts Tab at the top, find the contact you want to call and open their profile.
In the right-hand side you will see the contact's information. Click on the phone icon and select the action (Call or Send an SMS) that you want to perform.