Kids at the young age crave their parent's attention  and they haven't yet learned that there are times when it is not appropriate possible for parents to provide it.
From a child's point of view, you are suddenly unavailable and does not listen to him or spend time with him, he starts to feel left out. 

Why does my child interrupt all the time?
Interrupting comes naturally to children because they tend to forget that other people have needs that are as important as theirs. Even if your child knows that she's supposed to wait for a pause in the conversation and say, "Excuse me," the protocol slips her mind because what she has to say feels so urgent at that moment. Your child may be especially prone to interrupting if yours is a talkative family or if she often hears you and your spouse or other adult relatives finishing each other's sentences. You've probably already talked with her about when she should interrupt (the house is on fire) and when she shouldn't (she's bored). She probably also knows (even if she doesn't always remember) that the appropriate way to preface a necessary interruption is to quietly say, "Excuse me." But putting these principles into practice is much harder than merely comprehending them, especially for high-energy kids.

Offer your child plenty of positive attention as a preemptive strike. If you provide him with a dose of your undivided attention each day, he'll be less likely to exhibit attention-seeking behaviours, such as interrupting. Also, give him attention for positive behaviour, such as waiting patiently. When you know you'll be talking to someone for any length of time, plan ahead to keep your child occupied. If you're going to be on the phone for a few minutes, give him something to do and tell him the rules. Rub your child’s back to let her know you’re aware of her presence, but don’t make eye contact. “The message is: I know you’re there, but I need to finish my phone call.” Do give information, Explain to your child. Before you make the call You might say, “Daddy needs to talk to Grandma sometimes, and it’s not respectful for me to talk to two people at once. Unless it’s an emergency, you need to wait until I’m finished before asking your question.” An agreed-upon hand signal that indicates you’re not available to talk may also work well. Do make it possible to be close to you Keep a basket of quiet playthings or some stickers and a little book near the phone. Explain to your child that he is welcome to be near you, but not to interrupt you. Young kids have limited patience, so none of these strategies will give you the time and space for a long catch-up over the phone. Sometimes you may have to tell your caller your child needs you and you’ll call back.  Don’t blast your child when you get off the phone. It may take a few tries, but kids will learn not to interrupt you.A gentle talk about your expectations. Talking will only take you so far: It’s consistent and repeated actions, such as taking the call in another room, that help children learn that when you say you’re not available, you mean it.

It is the time to understand your child and help him/her to teach not to disturb you when you are doing something important or involved in conversation with others. Children need to understand that they can't be the centre of attraction all the time, sometimes the attention  is needed elsewhere also. But you need to gently encourage your child to understand this.