Today I got a new iphone4s from ATT and kept a 4 digit passcode to use that.
My friend is able to call and send messages using siri with out unlocking the phone. Is it a bug? Where I have to inform this?
Well I haven't tried that out my self, but i think that you will need to use the earphone to activate Siri without unlocking the lock screen of your device. I'm not sure if there is a way around it without using the earphone.
You can activate Siri if the phone is locked or even just asleep by holding down the home button until the mic pops up and makes the "speak now" tone. though Siri cannot do certain things while the phone is locked, i.e. open google maps and look for places.