I think the problem there is that a lot of people just won't bother. Some people would just put "pick up" on that subject line, or leave it empty (surely you wouldn't make it mandatory, nobody would accept that), or any number of things.

I personally hate unscheduled calls, and I'd love it if everyone sent a text first to check if you're free for a call, and only call after you've accepted, but... that's just never gonna happen.