> "Oh sorry, I happened to walk by an opera house."

It would be unfortunate for them to phone you precisely in the small period where DND was turned on as you walked past the opera house (even a large one would be, what, under two minutes of walking?) and not try you again.

(Because if you have instant location-based DND, yes, it might turn on as you walk by an opera house but equally it would turn off when you were past the opera house. Normally, though, I'd expect location-based DND to only kick in after a certain period of time in the same location - say 10 minutes.)