I can see both sides of "needing to answer calls at all times" and "keeping things separate". I have a customer who basically is probably 30%+ of my business. They got my cell phone long ago, and that's the only number they call, and they freak out when I don't answer. They aren't mad, but if I'm on vacation or something, I'm not answering their calls...PERIOD. I'm very protective of my cell phone...it's for MY convenience, not theirs.
That being said, I have my shop phone and my cell phone ring at the same time most of the time so I can grab whichever one I'm closest to and it helps when I am out of the shop and I do want to answer calls. I just have too many clients that are work-a-holics, and I'm not going to work on their crazy schedules.
I offer great customer service, and they know that, and when I leave town, they REALLY realize that, but for me, I've got to be able to separate business from personal. Ask any realtor that's got their cell number all over the place, they can't ever escape that phone...ever.