Your extensions should match your main greeting. If you have not setup a main greeting learn HERE.
Step 1: Add a Employee Extension
Navigate to the Extensions on the left hand menu.
You'll see "Employee Extensions" under department extensions. Select the "Add Extension" button.
A new extension will appear at the bottom of the list.
Step 2: Customize Your Employee 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: John).
On the right column you can add call forwarding numbers, set email addresses for voicemails to forward to, and add a voicemail greeting for the extension.