Attached is an example of an enterprise grade customer support desk data model, that you could customise and use in your application.

The complete data model includes entities designed for 
-Ticket support / Call logs
-Live chat support
-Online website visitor monitoring
-Task management
-Knowledge base, includes News item
-User survey 
-Multi company brand support
-User management^

You might want to have a look at it, study it, learn from it, get some ideas, do as you like with it :-)


Example: ticket support data model



Note: ^You could replace the user roles/group management entities tables (User*) since you can use the built in agile platform "Users" application to manage your application users.