Comment Not cheap... (Score 2) 141
But you could look and see if Jet is within your budget.
http://www.obsidian.com.au/products/jet/jet-isp-telco
At the very least a base install will give you some billing software and hooks for other automation. It wouldn't hurt to drop them a line, at any rate.
disclaimer: I used to work for obsidian ~6 years ago. they're a small company, but full of bright people and they have a lot of experience in the area. if jet isn't for you i have no doubts they can at least give you some honest advice on what to look at instead that's within your budget, fits your needs.