Sorry only just seen this. The offence is "using a hand held device" using means interacting in any way with it for the purposes of communication.. (there is an argument to be had surrounding this, however if you aren't caught for the technical bit of using there is still plenty of scope within the definition of due care for a prosecution) with regards to being stationary at lights etc you are in charge of the vehicle and therefore should be in a position to take evasive action (think emergency service vehicle approaching)
Really it's super simple. Turn it off. Put it in the boot. Lock it in the glove box. Whatever. Just don't use it when you are behind the wheel.....