This is where you come in.
Can you really handle it? If a guy calls you up and wants you to take a crap on his face, over the phone of course, are you going to respond by : a)Go "ewwwww, yuck!" b)laugh at the caller or c)go with it, "I'm squatting on your face right now, you are my human toilet!" If you're going to shun or laugh at the caller, perhaps being a PSO is not the job for you.
True,the scat calls are far and few between, but what I am getting at here is that if you're to be the ultimate PSO, you've got to educate yourself on the world of kink and fetishism, be willing to listen and respond to pretty much anything. Fetish and kink does not stop at Domination and Submission, and all it's accouterments. It is a vast world of various types of play and behaviors.
Now depending on who your employer is, it may be ok to not take calls you're uncomfortable taking, but that is between you and your agency. That all said, it is best to politely decline instead of chastising the caller for what you deem abnormal. Chances are they are calling YOU because they can't share their fantasies, no matter how twisted, in their real-time life.
The web contains oodles of information about various fetishes and kink. Educate yourself, open your mind. Being a superb PSO is not all about playing telephone blow-job queen. Learn what goes on beyond the realm of regular, "vanilla," heterosexual in & out sex:
Here a few good starting spots: