When you open Webex, it should take you to the first tab, Messaging. Make your way to the side tabs and click Calling, this will be the main function for using Webex remotely.
When in Calling, you will be taken to the "Calling" section where all your calls to your extension will be located. Next to that will be the number pad where you can make calls.
Underneath the number pad is a drop down menu where you can switch from your computer, My Computer and your office phone (Ex: LIB122 Vincent Chang x####).
Note: You will want to have this setting set to My Computer instead of your office phone. Doing this will allow you to actually hear the calls you are getting.