According to the conversation, Ruth calls Tim because

M: Hello. University Books. Tim Weber speaking.
W: Hi. Tim, this is Ruth.
M: Oh, hi, Ruth. What’s up?
W: Well, (4) the Student Federation needs a couple of volunteers to give guided tours to the new students during orientation next week. Would you be able to help out?
M: That depends on the days you had in mind. I’m working here full-time before classes begin. It’s really busy now, with all the textbook orders coming in, but I do have some time off.
W: What about Saturday? (5)Most new students arrive at the weekend.
M: Sorry, I have to work all day Saturday. How about Thursday or Friday? I’ve got both mornings free.
W: (7)I don’t have the schedule on me... Ken’s got it. Maybe you can set something up with him.
M: (6)I’ll only be able to spare a couple of hours though.
W: No problem. (7)I’ll ask Ken to get in touch with you later today. Will you be at this number?
M: Yeah, till for...Look. I’ve got to go. I have to get all the orders out before I leave today.
W: Ok, go back to your work. Thanks Tim. Bye.