There is a cell phone waiting lot at RDU, it just doesn't have good (any?) signs. You have to follow signs to rental car return to get there. See this link for directions. I just do what suthurn said, though, and park in hourly parking. First 15 minutes are free, after that $1/hr. If you get back to the ExitExpress kiosk within 15 minutes of pulling your ticket, it'll punch it for you with zero fee, then you have another 15 minutes to actually exit the parking area.