The general consensus would be to ring the escort. However, this has it's own problems. Firstly you need to ring her during her working hours and then if she is in the middle of an appointment she will not be able to answer the call. Leaving a short polite VM may not be the answer for many guys as they may not want the escort returning their call.
The PM system is another option but this also has it's pit falls. First off many escorts have not signed up to the forums so this option does not exist fr them. Secondly, for those that have signed up not all visit here everyday, in fact only a small percentage visit on a regular basis. Keep an eye out for those who post regularly or who are online regularly, these ladies usually reply to PM's
I tend to PM the ladies first. This has worked for me very well in the past. I have also picked up the phone and rang the lady and I was lucky in that in all but one time I got through first time and arranged a meeting at relatively short notice.
It should also be noted some ladies state on their profile that appointments made via PM will take priority. I asked one lady about this and she said that when she takes a booking via PM (she does insist on a conformation phone call) there is a lesser chance of a No Show!