Keeping it crotched is the best way. Also you can put the bottle in one of those big trucker type coffee cups and when you get the call just say you're not ready to pee yet and need to drink a little coffee. Nuke the cup for 10 seconds and when no one is looking transfer the bottle to your crotch. I prefer the crotched method because then you're always ready no matter where you are and you don't have to nuke anything.