Your cat is jealous that you are talking to someone else! Just kidding!
When your cat hears you talking on the phone, he probably thinks you are talking to him. Your cat doesn’t know what a telephone is, and therefore he doesn’t realize that it is actually someone else that you are talking to. So, when you begin talking, he assumes you are talking to him. When you start talking on the phone, your cat probably starts talking back to you by meowing. Your cat may also rub against you and try to get your attention in other ways.
Talking on the phone can be a great opportunity to give your cat some much deserved attention. If he thinks you are talking to him, there’s no harm in pretending that he is right. When you talk on the phone, feel free to multitask — you can pet your cat and scratch his head while you carry on a conversation with someone else. He will absolutely love the extra attention, and he’ll think you’re carrying on a conversation with him!