It sounds like you need to start looking for another job because your company sounds crappy. Like other posters said, complain to your boss en masse but with solid points on how this will lower productivity and how hiring a part-time receptionist would make the company run smoother. If this doesn't work I'd suck it up but actively begin job searching.
Originally Posted by multicultcurly
I agree. I would not be happy at all filling in for a receptionist. I'm assuming they are not being sexist about this since you haven't mentioned it. I would definitely start looking.