Honestly... it really depends on the person standing infront of me....
Looks like a linebacker in a dress... horrable voice... but is acting female? "Yes mam, no mam, so on and so forth..." I hope "SHE" has a nice day...
Looks female but sounds like a guy... but very soft spoken and so on. "mam"
Looks female but acts like a man in a dress, has a bad voice, guzzling beer, spewing obcenities, and scratching there crotch <Or worse> "Yes sir, no sir... You make it hard to call you mam, your not acting like a lady...
Looks and sounds female but dosnt act it.... Hey sounds like my ex... and two of my best friends... though there obviously female, there more of tomboys...
Looks, sounds and acts female... duh "mam"
Looks male but dosnt sound it "Sir"
Looks female but is obviously trying not to, and trying not to sound it either. "Sir" with some difficulties...
Looks female and sounds female... just with a short haircut or something "Mam"...
Looks, sounds and acts like a man. duh "Yes sir"
Sounds female/male on phone <reguardless of who they really are> go by the name given and hope you dont make an ass of yourself.... Some GGs sound like men, some GM's sound like women<more rare> and it does make a TS's day when you deal with them as there gender over the phone reguardless of voice. Years of customer service experiance at work

Seriously It seems to be on a case by case bassis... Where I live though, I generaly run into those horrable creatures known as Drag Queens... I do go out of my way to call them sir, they are not women, even for the day... One or two I make exceptions for, and any new ones I do as well... but yeah...