While I usually only see providers that are on P411, many of these now days also have a profile on Tryst. One of these providers was complaining that more people from Tryst reach out and then cancel.
I think everyone understands that cancellations at the last minute can happen, but always notify them that you can't make it. Just be honest if it might be difficult to make that time work if you are seeting a time up. I have said to just pencil me in and will try and firm up 24 to 48 hours in advance.
I've noticed a lot more providers wanting deposits, and the more people cancel the more likely deposits will be requested. I would hope that most providers would at least give you the first meeting without a deposit unless you cancelled, then a deposit if you wanted to make an appointment the next time..