Well, a good first step would be not to pick up the phone in the first place, unless you're sure that it's an important call. That's why they invented caller ID: with only a little effort, you can identify “must-take” from “can ignore” phone calls.
You can also invest in an administrative assistant, or make it a practice not to hand out your cell phone number to clients—or at least to lower-level clients who don't require 24-hour attention.
What you should do, though—and which too many lawyers don't do, for fear of offending their clients—is to calmly and courteously explain to the offending caller that your time is valuable and that every minute you spend on the phone is one less minute you can spend preparing his case. A reasonable client will not be offended by this explanation; in fact, he will probably be impressed that you guard your time so zealously, with his interests uppermost in your mind.
Another good strategy is to agree to talk to your needy clients, but only during certain times of the day, and only when the call has been scheduled in advance. When you make the appointment, you can tell your client that you only have five or ten minutes to talk—and when that limit approaches, you can explain that you have another call scheduled and that it would be unfair to the next client to cheat him of his phone time.
This is a time management problem. It's a known fact that lawyers who successfully manage their time are more successful attorneys and esteemed more highly by clients than lawyers who are constantly playing catch-up and showing up late for appointments.