A: At 4, your son is old enough to start learning the rules of the house. And politeness while others are talking -- whether on the phone or in person -- should be one of them. This is a great learning experience for you, too, since it won't be the first time you'll have to teach him about polite behavior. The earlier you teach him, the less trouble you'll have later.
When your son is in a receptive mood, simply state the rule: "When grown-ups are talking, even on the phone, you must wait until it's your turn. If you have a hard time waiting your turn, I will turn my ears off to you until I am done with my call. Then there will be a short time-out to help you remember your manners." Then practice with a pretend game -- take turns making phone calls and interrupting each other and getting sent to time-out. Let him see both sides of the issue. When it comes time to take a real call, do your very best to ignore his antics.
Make sure to notice his excellent manners when he is able to entertain himself quietly while you're busy, and make sure to give him the attention he needs when you're not distracted by the phone. He may challenge you -- setting a new limit often means things get worse before they get better -- but if you stick to your plan, eventually he'll see that you mean it.