If enabled, you can use the Feature Access Code (FAC) on your desk phone to answer someone else’s phone call or barge in on a call they’ve already answered. This feature must be enabled by your administrator. To enable for a user, see Allow users to barge in to other people's phone calls.
1

Enter *33 on your phone.

If set up this way, you'll hear a tone letting you know that the system recognizes the access code.

2

Enter the phone number of the person whose call you want to barge in to.

When connected, you're in a three-way call.