I been at my same job for a few years now.
Very seldom got called "sir" and just have not had any real bad experiences. I mean if I got called "sir" twice in one week, it was just a bad week.
Though I have not been lazy about my presentation (still do hair, makeup, use female voice, and dress well) it seems lately I get "sir" about twice a day lately. Of course it is always men who do so.
I don't know if it is because there seems to be this flood of talk in the media about transgender stuff lately and people are getting tired of hearing about it or maybe people didn't think about it before but now they do and are not sure how to react to us...
Often times I just say, "it is ma'am" and they are like, "Oh I am sorry..." in a sincere enough tone.
The ones who are tired of hearing about it - Do they maybe feel the need to challenge a label, kind of for the same reason a kid in a Honda civic thinks he needs to try to race every car that is actually fast? Or maybe how a weight lifter hears from pencil necks about some other guy they know who is 10 times as strong? Or how someone might tell a martial artists, "Oh but I know this guy who can kill someone with his pinky toe!"
Like peoples' precious egos are hurt so they have to try to put someone in their place? Maybe the general public had few problems before but now that it is widespread news that we exist, some folks just want to discredit our efforts?