Your extensions should match the instructions given during your main greeting. For example, if your main greeting says to "Press 1 for Sales", you should build Extension 1 and forward it to your "Sales" department. Your "Sales" department might be a team of people or it might just be one person. If you have not setup a main greeting learn HERE

Step 1: Add a Department Extension

Navigate to the Extensions on the left hand menu. 

You'll see "Department Extensions" and notice there is already an "Ext 0 - Operators" created. Select the "Add Extension" button.

A new extension will appear at the bottom of the list. 

Step 2: Customize Your Department Extension

Select the edit drop down menu.  

On the left column of the extension menu you can select the extension number and enter an extension name (Ex: Sales).

On the right column you can add call forwarding numbers (Ex: Bob's Cell Phone), set the email addresses that voicemails will forward to, and add a voicemail greeting for the extension or use the default greeting that is already loaded.