On a Partner, to park a call, you Transfer it to your OWN extension number. To retreive it, you dial Intercom 6 and the extension number that parked it. If you are using a bunch of 6-buton sets, you could program the sets to select intercom first, and the two unused buttons to be park and pickup. For park, the button would be "Left Interom and its own extension number" (and you might want to put a piece of black electrical tape over the LED so that you don't have to watch it light up when you are off hook on your phone), and for the pickup button, you would program "left intercom-6", which leaves the user to dial the particular pickup code. While the call is parked, the call will blink on hold at the parking station, and will ring back after a couple of minutes.
Also, if someone else with a Key Mode phone, who can see all the lines puts a call on hold and hollers for you to pick up line X, you would dial Intercom-6-8- and the two digit line number (like 01 for line 1, 02 for line 2, etc.) If you have less than 10 lines, I usually tell the customer the pickup code is Intercom-6-8-0 and the line (1, 2, 3, etc.)