Who is doing the cancellation? Provider or customer?
If a provider calls to cancel last minute with a decent explanation, I would try to book once more. If it is the agency, I would also attempt to book again. If an agency girl does an NSNC, I would expect the agency to provide some sort of accomodation. If the provider did a NSNC, I would post it on her review thread.
As a customer, I assume a NSNC would get me blacklisted by the provider or the agency. If I call to cancel at the last minute, I would hope to be given another chance but I fully expect the possibility of being blacklisted.
If I have seen the provider in the past and have a record of being reliable with her in particular (or the agency) I would certainly hope that my last minute phone call would be taken at face value.