@Phone tasks are in your total system? That would be the indicator for me. If the overarching system has 15 tasks in this context, I'd leave it all in one list and skim it before I jumped in the car. That would alleviate the need to look at two lists or nest them in the some way.
But if you have 40 tasks across these two scenarios, I could see breaking them out into two contexts. But I still wouldn't nest them. I've been learning that in my own system nesting is usually an indicator that I haven't defined clear lines between my contexts.
The break point on my end of when to consolidate and when to break them up is based on how quickly I can skim a list and know what to do in the situation. I generally try it consolidated first and if I sense it taking too long to determine what to do, I look for ways to break it up.
All of that said, I would also argue that you shouldn't be making phone calls in the car at all. Having read BrainChains, I can say that our brains can't focus on driving enough while talking on the phone (or texting) to guarantee your safety.