Volume Scalibility is the ability of a billing solution to scale to the required volume of calls, invoices, and web traffic a company will generate.
The call rating engine is one of the most important aspects of a billing application for Asterisk. The number of CDRs generated in almost all scenarios will far exceed the number of invoices, accounts, or web activity, and the number of rates against which the CDRs must be matched will determine the processing time required and test the efficiency of a rating engine.
The invoice generation is probably the second most important aspect of an Asterisk billing application. If performed properly, the process of generating the invoices should not be as intense a process as call rating, since there are fewer unknowns and simpler algorythms. But all the same, the number of invoices generated each month will play into the viability of any potential billing system.
Most billing systems will offer some type of web portal for the clients and staff/admins to navigate. While webserver loads are generally minimal compared to the time and power needed for call rating and invoice generation activities, the number of clients/accounts will become a factor as to the scalibility of a billing solution when those clients will have web access to a portal for ordering and/or account management/statistical purposes, such as reviewing call details, faxes, voicemails, and account settings.