What's the difference between a Main Line and Department?
Think of your Main Line as a net for all your inbound calls. Once those calls reach your business, though, your callers may need additional help from a specific group of users, like Finance or Customer Support.
That's where Departments come into play.
By creating a Department for a specific group of users, you can set up automatic call routing rules so that callers looking for customer support can automatically be routed to your reps (among other options).