Logically you ought not be, but I'm sure they'll have made the law so it's simple to apply and would nab you anyway. Obviously you are in control of the vehicle as you can end the call when the lights start to change, but you can be certain they won't have bothered detailing that sort of thing. that's be too reasonable.
Reminds me of an experience I had when I was a youth. I'd bought a moped and a short while later noticed there was something not quite right with how it sounded: so I sat on it at the side of the road outside my home and listened as I revved it tying to work out what was up. What did I get ? Some cop car pulls up along side and asks where my helmet is. I explained I wasn't riding the bike and had to have my helmet off to hear the engine properly check an issue. He just repeated the question, so I worked out he was a bit of a moron, put the helmet on, pretended to carry on listening to the engine until he'd cleared off and I was able to act rationally once more.