When you call, you will be given the number of the cab. Each of them has a 3 digit number painted on the exterior (front, side and back).
Make sure you wait for that specific cab. The assigned cab has to wait at the pickup area for a minimum time before they can leave (something like 1 hour).
There are a couple of rogue American Taxi drivers who try to short-cut the wait in the staging area and will try to say they are your driver. It hasn't been a problem for a while but it happened to me a couple of times about a year ago.