Observe people around you to find out what they ask when they have made a call to somebody: I'd bet that you will hear something like "Can you hear me?", "Where are you?" and "Can you speak now?" I've always said to my contacts that they are free to call me anytime, it's my responsibility to turn off the phone if I don't want to be disturbed but sometimes we all make mistakes...
When everybody was using fixed line phones people were merely making calls to some location and hoping that right person is present and available. Now at the era of mobile phones people are making calls directly to the right person but the location keeps changing. This is fascinating but also disturbing if you have travelled to other continent and you have just waken up to an innocent friend's call - how could he have known that you are at the other side of the world? Yes, how could he have known that? I made a quick search to find a solution that would allow me to share my clock with my friends; if I'd be able to see that somebody is at a place where it is currently midnight I would think again whether it is a good idea to wake him up just now. I didn't find such a solution.
This kind of a solution should of course integrate seamlessly to terminal's address book and updating the clock info must be automatic. The difficult part in this solution would be how to make service widely known and get all members of the contact list to share their information. Maybe this could be a part of services like Dopplr or some other cool information sharing solutions?