A2Billing is a great piece of call billing software for Asterisk. It can be integrated with FreePBX and used in lots of different ways.
I run my own Asterisk system with FreePBX and use a2billing to do least cost routing. It’s possible to import rate tables from many different voip providers and then let a2billing route the call based on the cheapest route.
One of the things you need to be aware of is that a2billing will route based on the ‘best matching’ rate. So, lets say you are trying to call +17061234567 and you have rate cards for 2 providers. If provider_a has a rate for +17061 at $0.02/min and provider_b has a rate for +1706 at $0.01/min, a2billing will choose the more expensive provider_a as the rate is a better match. This could require some manipulation of the rate tables to get things to work how you want.
As I said a2billing can also integrate with FreePBX so that you can pass all outgoing calls from FreePBX to A2Billing to allow it to do the routing. You also get the benefit of better CDR reports.
So, if you’re looking for something to do least cost routing for Asterisk then it might be worth checking out a2billing!