As per @Geektastic's statement regarding phone use I have been that guy who has honked my horn to gain the attention of the person in the lane beside me to tell them to leave their "f*******" phone alone and keep their eyes on the road. Other times I've actually reported phone use to the police and enjoy seeing people get pulled over for it. I've got Apple Carplay in my car and don't ever touch my phone when I am driving even when stopped or gridlocked in traffic.
I had a similar experience recently.
Approaching the Harbour Bridge, I was in the 2nd lane from the left. In the left lane, a little further up, this car drifted over into my lane and then jerked back into their lane. A short time later, the same thing occurred. Going up the Bridge, I passed the car and looked over as I did. She was holding her phone up right in front of her - clear as day. So I gave her a beep of the horn and I have no regrets over doing that. We made eye contact but I don't know if she put the phone down, as she was going slower than me and I lost sight of her.
Further along the motorway she went past me as my lane had slowed down to a stop. But this time she had the phone up to her ear *facepalm*
Of course, I filled out the bad driver form on the police website.