We have the same requirement and at the minute have 4201 programmed as a local speed dial on each device.
However - and somebody might be able to confirm this for me too - it looks like 6.2 includes this feature? We are updating to 6.2 this week - but from the release notes:
The MiVoice Office 250 Release 6.2 introduces a new phone key type, Park/Pick-up in the dropdown list in Keymap programming. A new timer, Park Recall is also added for calls parked with Park/Pickup Key.
The default value for the Park/Pickup Key is 180 seconds and the valid Range is 30-600 seconds.
The user can configure Park/Pickup Key for the following entities:
• Phone keymaps
• Programmable keys for a phone
• PKM modules
This new key type can be used to:
• Park calls
• Pickup calls parked by you, or call parked by another phone using an appearance of the same key.
• Pickup Group calls with the press of one button.
• Provide a visual indication of calls that can be picked up.
Which looks promising.