I'm new so I'm figuring out some of the things I keep reading about.  Some that are curious to me are the two call system and the providers who duck out for a few minutes.

I'm imagining the 2 call system is so that they can make you wait a bit if another appointment is running long, possibly it's to keep an eye out for you if there's trouble or something. Possibly it's also to avoid giving out their address specifics until they know you're actually there to limit who knows. Am I close?

Ducking out for a bit and leaving you alone I imagine is to verify  the donation, and maybe occasionally to give you privacy. Probably also so that you make the first move (getting naked) and if they come back and you're still clothed or have underwear on they can proceed with caution in case you're trying to bust them.

Am I somewhat understanding all this

...to let her know you are there. She calls or texts to give you the apt. number/room number and says I'm ready, come on up. (Sometimes she says give me five minutes and calls you back to say come on up. Lot's of variations in this.) That's the simple version.  

Some here have described it as more like a 3 or 4 call system. You call her to get the appointment, then call to confirm and get the address on the day of the appointment, then call when you arrive, then she calls to give room/apt number and give you the come on up. Again there is a lot of variation with this. There is of course texting instead of calling and you can do almost all of this by e-mail now.

Yes, her protection is one reason for this. It limits who knows her exact location. Some will even give you a close location and then give you more specifics when you call to say you are there. I've read here of providers having a location where they can see you at the corner and look to see you before they give out specifics.

I'm not sure what you exactly mean by the provider ducking out but if you mean they leave you in the room or take you to the shower and then leave you alone then they may be calling someone to let them know you have arrived. It's a safety precaution for them. Often agency providers are letting the agency owner/scheduler know you have arrived. Yes, it also lets them check the donation out of your site. Even when I ask them to join me in the shower they will usually come in a minute or two after I start.

Yeah, you pretty much have it. If you see the same providers on a regular basis you'll notice some change in this as they get more comfortable with you.

Remember though we often talk as if there is a standard operating procedure the really isn't a standard operating procedure as providers and mongers alike have their way of doing things and they have a way of finding those that mesh with how they want things done.

Just thought you might like to hear from an actual provider on this.    

As Xyz confirmed, we don't want to give out our address until the day-of and yes, it's so people don't have it till they actually need it.  

Yes, occasionally a previous date will run over the amount of time allotted as a cushion between scheduled callers.  
Not giving out the hotel room or apartment number until we're ready for our next guest assures that no one is made to feel uncomfortable in that situation.  
However.....what really is more likely to happen is that if I give my room number out to someone when he confirms, then he may wind up knocking on my door 15-20min early, thinking that's okay.  
Also, many ladies prefer you not to knock on their doors: they'll watch through the peephole for you and open the door as you approach.   This minimizes the amount of coming-and-going noise that can be detected by neighbors and housekeeping.  To do that, we need to know when you're actually on your way up.   First: I don't particularly care for the phrase you used (ducking out), as it suggests something sneaky going on.  
Please look up the definition of the phrase prior to disagreeing with me on that one.  

Perhaps it would be more correct to say that the ladies excuse themselves from the room for a moment or two.  

I've worked for a couple of agencies/services where that was the standard procedure: show the client to the bedroom, then leave the room to count the donation and check in with them.  With one agency we were to also then deposit the donation into a drop-box style safe.  We got paid out at the end of our shifts.  

Nowadays, my preferred way of handling this is to have my client set down the donation and then he is to go to the restroom to wash his hands.  That is when I check the donation, put it away, turn off my cell phone, etc.  
Some independent ladies even have a partner or other person watching out for them, and they check-in and out with that individual.  

With a massage-style place I worked for very briefly, we were to leave the room for the same reasons as above, but also instruct the gentleman to make himself "completely comfortable"......... there was even a peep-hole to check that he was indeed naked and either wrapped in the provided towel or already lying face-down on the table.  I never asked them why this was the procedure so your guess is as good as mine.  

I've only known a couple ladies who did this on their own: one, because she believed that LE wouldn't get naked so this was an additional "check" on top of the screening.  
With the other, it was her habit to strip down to lingerie (after checking & securing the donation) and then return to the bedroom: it just made her more comfortable if he was either mostly or fully naked at that time.  

 
Hope that all helps a little!  You're already on the right track by coming here to ask questions; please continue to do so!

xoxoxoxooxoxoxoxooxxo

If you already know her room or apartment number on a repeat visit, PLEASE still adhere to the two call system. The only thing worse than showing up 20 minutes late to an appointment in P4P is to show up 20 minutes early unannounced.

Besides running the chance of having her previous client still there and the awkwardness that would create, you also run the chance of her being in the shower, putting on her make up, taking a crap, or otherwise not being ready for your appointment.

If you do find yourself getting to her incall a bit early it is ok to send her a text telling her you "might" be a little early "if" that is okay with her, but if she prefers you will be there at the time agreed upon. Sometimes she may respond back to you that it's just fine if you get there a bit early, but the default assumption should be to show up at the time of your appointment, not early, not late, but on time.

END OF MESSAGE

If there is some thought that this somehow exposes an LE decoy, please disabuse yourself at once of this notion.

It does not.  

And if the guy is a newbie and doesn't even know what it means, then it's probably better just to tell him to get undressed.  I myself in my first go round ended up sitting around in my clothes till she made it a bit clearer

Good point