Theoretically, they shouldn't have to. If he's too busy, he probably didn't open it in the first place, leaving only one option.

In practice, however, I find people can usually tell via context that they're being ignored. There are some subjects you'd expect this or see it coming before other, more benign ones.

Exactly... People who are being ignored usually have reason to be. If you're too dim to realize I'm just busy, then I probably don't want to be texting you anyway. If you're in a situation where I might be ignoring you because you did something to perturb me, I'd rather you assume I'm ignoring you, because then it'll drive the point home.