Setting your presence while attending a WebEx meeting using your office phone number
When you attend a Webex meeting, you won't automatically see your presence change, so you will need to set it.
You have two options when joining the audio portion of a WebEx meeting:

I Will Call In
When you use your work phone or Jabber client, your WebEx audio will be over Cisco VoIP and your presence will reflect that you are In a meeting, on a call.
Call Using Computer
When you use audio that does not use your work phone, your Jabber presence will not show that you are on a call, Instead, your Jabber presence will only show that you are in a meeting (assuming that the WebEx host created an Outlook meeting using WebEx productivity tools). When you join a meeting this way you will still receive other inbound phone calls to your phone or Jabber client during the WebEx meeting. A workaround to prevent inbound calls is to update your Jabber presence to Do Not Disturb.