Freeside is an open-source billing software targeted at ISPs and VoIP service providers.
I have worked with the Freeside software and supported customers on its usage since 2011 and can provide support with updates, configuration, training, and more.
There is commercial support available from the creators but if that doesn’t meet your needs I can help you with a’la carte serviced billed by the hour.
I am also working on some updated documentation that you can find at https://www.nkent.us/wiki/index.php/Freeside_Documentation
Please visit the Contact page to schedule an appointment to go over your specific needs.