It's very cool. But think about the following scenario:
Alice answers a call from Bob.
Alice transfers Bob to 6001 and hangs up.
Bob is now in parking stall 6001, hearing MOH.
Alice calls Charlie and tells him that he has a call parked in 6001.
Dee call in and answered by Alice # at that time, Alice don't know the lot is empty or not
If Alice transfer Dee to 6001 # then Dee will talk to Bob, that's not expected
Charlie calls 6000, waits for prompt, then dials 6001#.
Charlie Listening to music.............
Event it's rarely happen, I do think Alice would like to now the if lot is empty or not before transfer. Make sense?