The guys from my favorite Irish radio show do the odd prankcall. While I am not always appreciative of those and sometimes even find them embarrassing, I loved this one. You gotta hear it for yourself, it is pretty funny and if I was the person working the phone in that store I would have hung up earlier, I can tell you that much :)
My favorite part is when he asks about the fnorty fnor carat silver ilver ring and can barely contain himself because he is laughing so hard. Really funny.