I let people in the office know that from such a time to such a time, I'm not to be disturbed because I'm usually working on something that requires a lot of concentration.
If that's the case, I don't answer my phone unless it's a family member. I let it go to voice mail. We have a $25,000 voice mail system. That's what it's for! If something is really urgent, the person has my cell phone number. If they don't have my number or leave a message, then it's not that urgent.
Anyone who has the option of not answering their phone for a certain length of time should take advantage of it. They will be amazed at how much work they can get done!