Why is he so awkward on the phone?

I met this guy online a month ago. We both have very busy schedules, so we have only talked on the phone a few times. We text all day, everyday. He expresses that he is interested, but when we speak on the phone, he seems like he doesn't have much to say. It is hard to keep the conversation flowing. How can he text me all day and then have so little to say on the phone? We are about to go on out first date this weekend. I'm worried it is going to be even worse in person! Someone help, please!