If you really don’t have something to say and don’t know how to talk to girls on the phone, you should at least be kind. If she is saying something like “I just came from work! I’m exhausted and stressed because of it”, make sure that you say something kind or something that can make her feel good like “Don’t worry, I hope everything works out”.
After saying that, make sure you can make her get her mind of work and say something that can make her feel better. You can talk about anything. When it’s the first time you call her, be sure that you ask about her hobbies or what her occupation is. These simple questions can be a fast conversation starter, especially if you find something that you both have in common. Be sure that you ask the questions and always talk about her instead of just saying things about yourself.