Our business currently has several applications that manage telephone calls for our customers, and each of them has its own logic for determining how much to charge for calls. We want to have a central service that will deal with all the charging logic instead, and our team has had a go at writing this. However, we don't have much experience with Scala, and need to get it reviewed before it can go into production.

Write a review of the code base, specifically commenting on:

• What you like about it.
• What flaws you can identify and why you think they are problems. What would you do to improve them?
• If you could only choose one improvement to make, what would it be and why?