Is it illegal to call a hotel and request someone else's hotel bill?

No, there is no illegality involved as long as no false pretenses were made--i.e. as long as you did not actually misrepresent your identity. If you did, then it's possible you could incur one form or another of either civil or criminal liability, depending on the exact circumstances. But if you simply called up and without misrepresentation asked for the information, then even if the hotel may have acted improperly in giving it to you, you would not have acted improperly yourself--people may *ask* for all kinds of things no one should show or tell them. Where there is some ambiguity is the case of your passively tricking or fooling the hotel--acting in a such a way that they would have concluded you were a party other than the one you are. In that case, again depending on precise circumstances, it is conceivable you did something to incur some liability.