FirstData also known as Linkpoint is the largest processor in the USA. You signup for a merchant account and you can use their API to process payments from your site. All I did while PayPay was "on the blink", I wrote an interface for my customers to make their payments. Only verified customers can get to any of the payment methods we employ, so it cannot be seen by the public. You have to login to access it.
Can you please list the steps that you used to integrate the gateway into A2Billing? Did you use the customer interface that comes with A2Billing, or did you write your own? I'm looking into doing the same thing, and would really appreciate the help! Thanks!