The unanswered phone can turn an ordinary errand into a whole afternoon’s uncertainty.

Waiting on someone else is more expensive than the task itself. One unanswered call means you cannot decide whether to go out, start something, or make other plans, so an errand that would have taken twenty minutes occupies four hours of half-attention. The cost is not the errand. It is the inability to commit to anything else while it hangs.

If you are the one being waited on, reply with a time even when you cannot reply with an answer. And when you are the one waiting, set a point at which you will proceed anyway, then go and do something that does not need permission.