Hello Razor.
I have been able to do it. I have the following configuration
Box1: A2B+MySQL+Asterisk+AdminUI
Box2: Asterisk
Box3: Asterisk
Box4: Asterisk
Box5: Asterisk
WebSite: CustomerUI
Box1 was the classic A2Billing Box as most of us know it. But Box2, 3, 4 and 5 have only Asterisk and a copy of a2billing.php agi script.
The only boxes which a "required" to have asterisk are the boxes you are going to send calls from (through termination providers) and the box that you are going to receive calls on (from DID providers).
For the location of the main a2billing installation, we had no choice. Due to the way a2billing was designed, we had to have the main AGI running on Box1 where all the inbound DID numbers are being sent.
You can have MySQL (or Postgres) on it's own machine and the web server with the Admin and Customer UI on their separate machine too or the same Box as MySQL.
For the CDR, we could not distinguish between CDR sent from Box1, 2, 3, 4 or 5. So we have left all the CDR on their own machines for the moment. But you can use CDR Custom to add custom field to the CDR if you want.
I have recently installed the MySQL CDR modules for Asterisk and it seems to Work Well. So we are going to simply send all the CDR at the same location (in the Database) cause even though we have many asterisk server, we want it to look like a simple huge server from administrators point of view (just 1 CDR). On the other hand, by hacking a2billing, we can have it to write the server name in A2B's CDR. Unfortunately, that's just a partial solution, cause multiple server topology in A2B only works for callback. Other modes have not been design to dispatch call to other servers than localhost. That is even more true for the 1st leg of a regular inbound call coming from a DID providers. There is virtually no way to decide to re-receive that inbound call on another Box if Box1 is overloaded.
That's why we are starting to check SER out. After thorough reading, we have decided to go with SER. And despite what people are saying, we believe that A2B can work with SER without any major modifications. It will save our lives.
PS: In addition, if all the boxes are behind a firewall, you will need to assign a different port for SIP, IAX and RTP Streams for each asterisk box just to be on the safe side. I am sure there is more but I can't recall it all.
Does that give you a better idea of the situation?
Added after 8 minutes: I have just remembered. We are configuring a FastAGI server and A2Billing will be called directly from remote Asterisk Boxes without having any a2billing on themselves. And we will configure Asterisk Realtime and use it partially. We will keep some provider settings specific to some Asterisk Boxes, but All extensions and configuration that can stay common to all boxes will be pulled from the same database.