Hi
You have a Star2Billing installation, thus set up a trunk in FreePBX and test it, you are probably not going to need much more than;
Code:
host=net2phone ip address
type=peer
Call the trunk net2phone, and test in FreePBX to make sure it works without the complexity of A2Billing in the way.
Once it works, create a trunk in A2Billing, put "SIP" in the Technology type, and net2phone in the IP address field. Use the rate simulator to check you have a rate for the destination, and that you are using the correct trunk, then test an outbound call.
Joe