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.
Enter the phone number of the person whose call you want to barge in to.
When connected, you're in a three-way call.
Barge in on Someone's Phone Call
If your administrator has enabled this feature and the person who's call you want to barge in to allows it, you can answer their calls and even barge into an ongoing call.