To my way of thinking, the two call system is typically used for incalls.  The first call is made an hour or so prior to the appointment to determine the general location of the incall (i.e., hotel, nearby intersection, etc.). The second call is made after you arrive in the general vicinity and within a few minutes of the appointment to get a specific room number, address, etc.  Hence, the value of having a cell phone.  If you think about it, this is a safety procedure that benefits both parties, not just the provider.