Company Access Controls
To make WHMCS really complete I think you really need to add in company wide access controls. So the client is the director of a company, and within that company there are levels of access rights 'contacts' as you currently have. But these access rights can be better set.
Currently, if a company director wishes to see all communications on the whmcs support ticket system, they have to be a contact. The only issue is, the director might raise a personal ticket to the whmcs admin user which is only to be seen by them and not the other employees. As a contact, currently other employees can see the tickets that are opened by the director. What would be really good is if the director / ceo can see everything including all domain names and services (or if you select what they can see), then the managers beneath them can only see their tickets (and invoices, services etc if allowed), and the employees beneath them can only see their tickets nothing else, but the managers can see their employee tickets. This diagram attached shows it a bit better.
In the setting of WHMCS this could be set as the client type, according to a company structure.
E.g. you could set up the first client as the director, then add 'contacts' but set the access level of that contact to what they can see of tickets, a bit like you can of invoices and other services. Submitted by OJE.
I think this addition would really make WHMCS the complete solution as clients of mine ask why this can't be done all the time!
Remember this is just the client structure and what different clients at the same company can view of the ticket information and service information.