Granted we have back-up options to our wake up calls but by offering the service the hotel should accept the burden of responsibility.
It can sometimes be a little more difficult to decide whose to blame. Recently I had a wake up call scheduled for a very early morning flight (from an airport hotel) and a back-up as I don't like to put all my eggs in one basket. My back-up woke me and I wondered what had happened to the call so before checkout I mentioned it to the manager. He saw that it was logged but I disputed receiving the call so I asked him to test it for himself...turns out the previous guest had switched the ringger off, so we were both in the right. Luckily in my case their was no harm but it could have turned out badly.