About 50% of inquiries are like this, in my experience.

I prefer to deliver (it solves a lot of problems including this one) but if I have to have them come here, I try to remember to give them an opt-out, so I say something like "call me when you are about to leave" or "call me when you know what time you will be coming", and make it clear that if they just turn up without calling first I might be out. That way they have to make the effort to phone you back and if they are just wasting your time they probably won't bother.

Liz