"The two surest ways to get your kids' attention: closing the bathroom door or dialing the phone." (someecards.com) Pretty much sums it up!
Put together a box of toys, games, and books that your children are only allowed to use when you are on the phone. Make sure these ideas are saved for 'special' times (when you're on the phone, or can't give your child your full attention). Also, load up on popsicles. It always keeps them quiet for a few minutes for an important phone call.