I am on call 24 hrs a day, the phone is never shut off. That being said, I do have rules.
When I am with someone else, the phone is always on vibrate. If we are discussing sensitive issues or my attention needs to be focused on you 100%, then it is placed face down - I can still hear it buzz, but I refuse to answer it; I am there for you, not the other person.
There are times when I am expecting an important call, if that is the case I apologise to the other person before we meet and tell them there is a slight chance I need to take a call. Still the phone is on vibrate but now face-up. In those cases the only call I take is the important one, and I excuse myself as soon as I answer.
On my days off, my phone remains on but all calls go to voicemail. I check, if it is urgent, you get a call back, if not, you are called when I am back at work.
I have always lived by the philosophy that my phone is there for my convenience; so I answer it when it is convenient for me and respond accordingly. The only reason I have a cell is because of the church. If I wasn't on call 24/7, I don't know that I would have one - they are so intrusive.