Before adding an external number, you need to create a SIP Trunk. Creating a SIP trunk can be found on the page creating a SIP trunk.
How do I add an external number?
Go to Numbers.
Click on the External tab.
Click on the Add a number button.
In the modal window, in the Add an external number field, enter the number that belongs to this SIP trunk user. In the SIP Trunk dropdown, select an available SIP trunk.
Click the Add button.