The PBX Center application includes the Call Part Center functionality.
You may want to "park" a call, so your peer will be connected to a music-on-hold application, while your device will be disconnected from the call. Then you or some other user can "pick" the "parked" call, i.e. to connect to the peer of the initial call.
The PBX Center application offers an unlimited number of "parking queues", and it provides quick access to the first 9 of them.
To park a call in the queue number n, connect the peer to your PBX Center function 7n by transferring the call to the 77n address.
You and other users of your Domain can park multiple calls in the same queue. The calls will be picked up in the same order as they were parked.
To pick up a parked call from the queue number n, you or other user of your Domain should connect to your PBX Center function 8n by dialing the 78n address.