All you have to do is dial 52# and you are automatically connected, however The intercom can only be accessed from the phone at Guest Service, and the phone on the operators desk. If you try to dial the extension from any other phone the system automatically hangs up on you.
Apparently this isn't the case, because in this guy's video he uses the red phones in the vacant checkout lanes.
I know for a fact that the courtesy red phones throughout the store (intended for customers) won't work, but haven't ever tried from the checkout lanes.
*edit: forgot to include the link to the video: