Your wife is being defensive because the tone of your questions is accusatory, even though your intentions may be otherwise.
My mother was this way and it made it hell to live with her as a teen. She lacked diplomacy. You may, too.
Focus on the reason for your questions: When you say, "Why haven't you sent me a text?" are you perhaps thinking, "I love it when I know you are thinking about me, and when you don't text me, I feel like you aren't thinking about me." If that's the case, then TELL HER what you are really thinking, and forget the question altogether.
Rather than saying, "Why is your phone busy?" you might say, "I tried to call but the line was busy. I was just missing you and wanted to say hello." See? No question. But you've opened the door for her to say, "Oh, I was talking to (name). I'm sorry I missed your call." And if she doesn't tell you who she was talking to, so what? She IS an individual and is entitled to a life that only includes you but is not LIMITED to you. Understand?