If all calls to a particular extension route immediately to voicemail, please read the following article and make sure your phone is registered.
If your phone is registered, please make sure you have the correct extension associated with the device:
Flip the physical phone over to the back and find the MAC address. It should be 12 characters made up of the letters A-F and the numbers 0-9. Polycom phones begin with 0004F2, and Aastra phones begin with 0008
Write down the MAC address
Click on Users/Extensions --> view extensions
Locate your extension in the list and hold your mouse over the glowing green circle
The SIP Address should match the MAC address that you wrote down. If it does not, the phone you are using is not the correct extension.