Depending on if the scheduler works for the MSC or a scheduling company, there are a few ways to approach this.
If it is a scheduling company, you could end up burning bridges with many companies if your perceived as being in the wrong.
If the scheduler is for a single company, and you are certain that they are in the wrong, you can almost always reach someone above the scheduler or get the information of this contact by calling the sales contact number of the MSC.
If it is a small operation, you will likely get the MSC owner. If it is a larger MSC, you will get a sales associate that is much higher up the organizational chart than the scheduler and can transfer you, or at the very least, give you the email of the scheduling manager. At this point you are being confrontational and expect to either have the scheduler reprimanded or dismissed or yourself removed from this company. Someone will get the short stick and if you don't have documentation to back up your allegation, expect to never work for this company again.
Unless there is much money on the line, I'd just let it go and not work for this scheduler ever again and they will quit when they can't fill enough jobs. This is how a bad scheduler is weeded out.