I just saw this in the news today:
Unlimited calling and texting for US$50/month. It's a race to the bottom with all-you-can-eat plans.
If you're charging per minute, per sms, per byte, the question is "why?". Save yourself a lot of money and quit billing for the core service! That Intec Billing Engine you're thinking of buying? Get rid of it, or use it for something other than charging for calls.