It doesn't matter. All phone calls would be important but also completely unrelated. It's not like he would be calling press and then the next thing he would be calling his wife and then somebody else, etc. just because he is in @phone mode. You see, this is surely efficient but ridiculous.If Steve was following the GTD principles properly, he wouldn't have that problem, because he'd be doing his weekly review and the only phonecalls on his @calls list would be ones he really needed to do.
He was not choosing available actions from contexts, he was focusing on the results and switching contexts whenever necessary. I think this makes a lot more sense, especially when all your contexts are always available.