Call Parking for Grandstream GXP-21XX Phones

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Concept
Single Button Call Parking is a feature that allows a person to put a call on hold at one telephone set and continue the conversation from any other telephone set.
The “call park” feature is activated by pressing a preprogrammed button (usually labeled “Park”) on a Grandstream device. This transfers the current telephone conversation to an unused extension known as a Call Park Orbit and immediately puts the conversation on hold. (This is called parking the call, and the call is said to have parked onto a certain extension. Essentially, call parking temporarily assigns an extension number to an incoming call.) Callers placed into a park orbit will hear Music on Hold until the call is retrieved.
Once a caller has been parked, a BLF line key on the Grandstream phone will flash to let other users know that the park orbit is occupied. Any user may retrieve the caller from the park orbit by pressing the BLF line key.
Definitions
There are two types of call parking orbits supported on the SkySwitch system. This guide covers Directed Call Parking using the BLF Key on a Grandstream phone.
Directed - This is the traditional call parking feature where the person that performs the park simply transfers the caller to a Call Park Orbit. The orbit (also called a queue) must be pre-existing and must be empty in order for the parking attempt to work.
Dynamic - In this scenario, the person that performs the park will dial *** and the system will transfer the caller to any available orbit (the parker does not need to know the location of the orbit or if the orbit is available or not). After receiving ***, the system will announce the orbit location.
The call park extension will only be announced when entering *** as DTMF. Due to how the system works, entering Transfer + *** will park the call, but won't play the message.
Creating the Call Park Queue
The first step in configuring Call Parking is to create one or more Call Park Orbits.
Create a Queue
Browse to the Queues page, and select the Add Call Queue button.

Configure Queue Extension and Type
In the Add a Call Queue screen, enter the following values then click Add:
Name - can be any word that is descriptive of the purpose.
Extension - For Call Park Orbits, the queue extension MUST be between 700 and 729 (Queues in the 720-729 range have Ringback enabled). This number range is reserved for Directed Call Park Orbits in the system.
Type - Select Call Park from the drop-down box.

Verify Call Park Queue
Confirm that the queue you created is listed on the Queues page.

Set Call Park User's Ring Timeout
After the queue has been created, manage the queue user and make sure Voicemail is NOT enabled. If Voicemail is enabled, parked callers will hear an option to leave a Voicemail at intervals that match the "Ring for" value. 
For Call Parks on the 700 - 719 range, we recommend setting "Ring for" to the highest possible number, 7200 by default. 
For Call parks on the 720 - 729 range, "Ring for" decides how long the caller will stay parked before the caller is returned to the person who parked the call.

Configuring a Call Park Key
The Call Park Key on a Grandstream phone may be configured using the device's web UI, or by remotely provisioning the phone using overrides in the SkySwitch Provisioning Server (NDP).
Using the Web UI
To configure a Call Park key with the web UI, browse to the phone admin page and configure a Line/Memory Key under the 'DSSKey' tab with the following parameters under Settings > Programmable Keys.
MPK Mode - Monitored Call Park
Description - Your description
Value - Parking lot extension

Transferring and Retrieving Calls
Once the queue has been created and a Call Park key has been configured as above, you may park any active call to the queue by pressing the key labeled Call Park. Once placed into the queue, the LED of Line Key 1 will turn red, denoting that a call has been parked.
To retrieve the call, any user with that Call Park key can touch the key to pick up the call.