From my view, just because you are in possesion of a company owned phone does not mean you are obligated to respond to work related correspondence at all hours of the day on any day of the week. This level of connectedness to work was never expected with the introduction of the beeper or email, so it shouldn't be expected with a mobile phone.
When one is hired for a job, they are given guidelines as to what responsibilities are expected of them, usually also including working hours. People are human beings and require time for obligations outside of work including to family and themselves. Time to escape from work is essential for workers to live healthy lives and continue to be productive. If they are constantly responding to a mobile phone, they lose that escape.
An exception of course, would be those who work in emergency services such as emergency surgeons, nurses, police officers, etc. as their work contracts sometimes specifically state that within particular time frames they must be completely available at a moment's notice. This; however, is due to the nature of their work and the fact that people's lives could depend on those workers being available.