I work from home, and a lot is on the phone.
A good investment is a handset that has a mute button, and preferably the kind that has a light so that you know it's on.
Best to prep first so that the kids, know not to interrupt, rather than do it whilst you have a client on the phone.
Also I've found the garden can be a good place to have important conversations.
One can run a phone cable a lot further than you might think...
5yo DS is too young to fully understand what I do, so I've equated headhunting with being a pirate
He understands that since I need to be a big fierce pirate that people are scared of, it undermines the terror I need to inspire if they can hear a little boy in the background
When he's been around when I'm on the phone, I praise him afterwards for being quiet.
One day soon, I'm going to get a recording of businesslike office noise so that I can use it to drown out "household" noise. My PC will be rigged to play it whenever the kids get a bit loud.
Also you ought to look at Skype or VOIP if you're on the phone a lot.