Ahh,
Another telstra competior, its all good.
Depends, but do you really need a billing system?
3CX currently has no interface to a billing system as yet, so here is your trouble. How are you going to use it for on charging you calls?
Few ways to go about that.
1. API etc for the billing system of your choice.
2. XML out put or something like that which you can import into your billing system.
3. The back yard job, and that goes like this.
Determine how you want to bill, eg by call, by call duration, by call destination etc.
Generate a report from 3CX and use CSV to create it.
Import your CSV into excell
Configure the spreadsheet to import into your bookkeeping application.
Now this works on a small scale, and can work with MYOB type stuff. But to do it on a large scale that will be different.
I am not sure what your setup is but, looking at your webpage you think BIG which is good. But in that scenario you might have to think about how much you are willing to risk with GNU type software.