There are so many AMPs in NYC but they all seem to know each and there is a lot of moving around.  When the phone girl from "A" quits and goes to work for "B" she brings her lists of safe phone numbers with her.  (That is, guys who have used services at at least one place and are, in general, not trouble makers or bad to the girls.)  There is so much moving around that sooner or later, it seems that every AMP knows every caller and which callers are OK and which are LE and which are not yet verified.  Maybe a three month lag time, at most?

I have read that some newcomers have trouble getting into an AMP if they don't recognize the caller's number.  Sometimes those guys have to show IDs at the door, answer some personal Qs, and even that might not get you in. It seems to depend on the amount of LE heat the AMP thinks is going on.  During crackdowns, nobody new is allowed.  AFTER the elections, it's another story.  

I read a story in the NY Times YEARS ago (1990-somthing) about an AMP near Times Square where the cops showed up as customers (or to hang out, NOT to bust anyone) all the time.  There were so many blue uniforms around and going up and down the stairs that it affected their normal business so they opened a second location right across the street.  One location was off limits to the cops, the other wasn't.

Bottom line: One AMP might take another AMP as a reference if you want to get in but I don't know of any indies or agencies who take AMPs as references