I don't know what to talk about by calling and texting my boyfriend.

Find your similarities. If you really don't know him, ask his friends what he likes, such as sports, diet and even dressing style. Find your hobbies on the basis of understanding, and you will have a chat. Besides, you can cultivate more interests according to his hobbies. Of course, you'd better tell him your hobbies and interests and you can create similarities together.

I have only known him for 20 days. Wouldn't it be better to get to know him gradually? If he doesn't take the initiative to talk, you can talk about a topic he is interested in. If you don't like this topic, he is long-winded, you'd better try to bear it, or simply communicate with him. The best skill for two people to find a way to fall in love is to communicate well.