I will often call a customer of mine and inform them I will be running late 20-30 min before I am expected to arive. I can't tell you how much they appreciate that little extra even if I didn't say much on the subject. If I were to just arrive late (say 20-60 min) without a communication preparing them, I doubt they would be my customer for long.
Yes, but that customer doesn't call you back in 10 minutes (and every 10 minutes after that) asking for another status update.