Most of the managers I've talked to are very cognizant of the fact that they can be held legally liable for saying "Nasty" things about a former employee. I think things work a little differently than you suppose. The way to "get back" at an employee is to either not provide information about the quality of the work, or provide information that gives faint praise.
So instead of saying either (a) you suck; or (b) you rule, the manager will simply say (c) yes, you worked here; or (d) I don't have time to discuss this. In this age of legal liability, (a) is not really an option, but (c) & (d) can easily be interpreted as meaning the same thing for those looking for subtle clues. In not going out of their way to talk about an ex-employee, the person on the other end of the phone reads between the lines and builds their own impressions.