I arrived at a booking once, while she was in the kitchen getting drinks she received a text, someone from the Agency would be there shortly to collect her Agency fees, there I am naked on the bed waiting for this girl to service me, she apologised after he/she had left, didn't lose any of my time, but I did suggest that she should have notified all the agencies she was with that she had a client at that time, I couldn't complain to the Agency I booked her through as despite asking she didn't tell me which agency they were from (she was with about 4 or 5 London Agencies)
So, not only does it happen before you get there, it happens while you are there as well