>Sit absolutely still staring teacher in the face, dead-pan
>Wait about 2 seconds and shrug, smiling
>Move phone to ear and kill sound/app/turn off/whatever
>Immediately 'answer' phone in one slick movement with aforementioned step
>Look away while having imaginary conversation. If interrupted, direct out-stretched arm and hand at face of offender
>Walk out of the room casually. Return 20 minutes later, giving zero fucks.