I need someone to explain why. What's so difficult with "I'm sorry, I'm not able to do that" or "I'm going to be late", "I'm fully booked", "I need to reschedule" etc etc. How long do you give them before moving on and finding someone else?
I have had more than my fair share of trades just not turning up, making false promises on work then not fulfilling it. They're all so unorganised and terrible communicators. It's so infuriating. Just tell me!