Stereo headphones without an inline mic still works for calls

[supergadgetman]

Sorry if this has been posted but I just have a quick observation. I picked up a pair of Nike branded Philips in ear bud style stereo headphones. They sound damn good btw. Really good bass response. And they are heavy for such small headphones, but there is no fatigue on my ears since they have built in adjustable over the ear clips. Anyway I was listening to resco radio and a call came in which I answered and the other party was able to hear me. And my TP2 was in a case on my hip. I know these dont have an in-line mic so either the mic on the TP is not deactivated when you plug in stereo headphones or it re-routes the mic to one of the earphones and keeps stereo output if thats even possible. I'm a lil confused here anybody care to chime in?