I usually say:

Hello! My name is (my name) and I'm calling for (provider name).

After a polite exchange of greetings, I ask if the lady is available for the time frame I am interested in.

Be sure to have your reference information handy....

When doing so, I more or less act as though I am responding to a personal...give name, occupation, physical description of self, how I heard about her, and a proposed time to meet.  Also will provide references, and any other info she requested in her ad or website.  By doing so, I give her the option to screen and decide at her leisure, rather than putting her on the spot with a phone call.  After she responds, which historically for me is generally positive, then I will call for the get to know more about you conversation, and to discuss details about the upcoming meeting.  This approach has worked well for me, though many of the ladies are just as cool with calling for first contact.  Just be sure to provide whatever info she requests, and try not to sound nervous.  If you have a good phone manner, I suppose you should use this to your advantage.  Good luck!

have a specific date, time and place (if outcall) to give the provider.

This makes the call that much more real to her and will likely get you an appointment as long as your references, etc. pan out.

Just remember, you only get one chance to make a first impression.  Bring your manners, don't say anything about $ or sex.  Get VIP and read the reviews; you will find all the information you need in her reviews.