When I was working, my star time was 0730, and I usually left the house about 0630/0645, hit the donut/coffee shop on my way in, and got to the yard about 0715, time started at 0730, unless the dispatcher called me a trouble call before I got to the yard, and my time started then..........if I got called on the way home, (quitting time was 1700) time stopped when I got home!!
If I was off and on call, and got a call, time started when I got the call and stopped when I turned the key off, when I got back home..........
Right after I retired, they activated the GPS on the trucks, so it might be different now, but because of a budget crunch......Tmen can only take the truck home when they are on call........so I'll bet some of them 2 hr minimum call-outs now, might be 3 hr call-outs!!!