In Germany it is common that your hotel room booking is guaranteed until 6 pm. If you plan to show up later, you can give your credit card number beforehand, otherwise your room might be given to someone else. The advantage is that there is no cancellation fee if you don't show up.
Sometimes it happens that I haven't given my credit number beforehand and on my way to my destination it becomes clear that I won't make it before 6 pm. I usually call the hotel and tell them that I am on my way, that I will show up and please do not give my room to somebody else.
My question is: Is this commonly accepted in hotels and will the hotel staff take it into account? Or could I just as well not call and hope for the best?