In Gmail chat, you can allow people to see your online status. The same should be available for cell phone users.
All mobile phone users should be able to update their current status in real time, such as "in bathroom." There should be entry time displayed when the status is typed.
This way, the user can have a comfortable, relaxed bath even while waiting for a important call. If you don't want to be disturbed for 5 minutes, you could type:
"In bath. Wait for 5 minutes.
- entered at 3:05 pm on 14.03.2011"
This would make our life a lot more relaxed, while improving communication.