Hi
There are a couple of ways of doing this.
At it's simplest is below, but you may want to make enhancements.
1. Every one dials 9 for an outside line.
2 Create your four SIP trunks in FPBX then A2Billing.
3. Create your 4 call plans, one for each customer as you want to use a trunk each. - You could just do one SIP trunk and one callplan for everyone.
4. Create a context in extensions_custom.conf as follows: -
exten => _9.,1,Answer
exten => _9.,n,Wait(1)
exten => _9.,n,deadAGI(a2billing.php|1)
exten => _9.,n,Hangup
5. Create 4 accounts in A2Billling and and note the account number
6. Create 4 FreePBX extensions and put the account code number in the accountcode field.
7 Test
Alternatively, you could look at this,
http://forum.asterisk2billing.org/viewtopic.php?t=2127
But rather than modify Freepbx extensions.conf, put the code in extensions_override_freepbx.conf
If two contexts are named the same in Asterisk, then the first context will take precedence. Anything in extensions_override_freepbx.conf will be the first context.
Joe