I've used the cell phone lot several times. I've found that the best timing is: passengers call you just as they deplane. By the time you arrive at their Level 2 [Baggage Clain level] Arrivals, they should normally be waiting on the curb for you!
Note that I usually pick up at the A side (north), so pick ups at the B side (south), so the amountof time may vary.
SWR
