Well they can expect what they like. I will do what *I* like, and I don't answer it unless I want to. I actually have two, my personal one and a company issued one. The latter I never, ever, carry except for the one week every two months I'm on call, and I leave the ringer off then, but that's for special reason. Even three years after I took this job I get calls from bill collectors for a former owner of the number, so many it would drive me crazy to leave the ringer on. I actually just posted my personal cell on the work call outs page. No one abuses it. Some colleagues have it in their phone books but only use it when it's justified. I carry the work one when I'm on call because I can still get work email through it more easily than through my personal one.
As for the personal one - any number I don't recognize I don't answer. They can leave a voice mail. If I want to call them back I will and if I don't I won't. If they don't leave a message they didn't want to talk to me very badly.
I really have trouble imagining life without my smartphone, but you have to be smarter than what you're working with. Since I started with a Star Trek reference, to quote Spock from TOS, "Computers make excellent and efficient servants but I have no wish to serve under them."