https://github.com/Star2Billing/a2billing

A2Billing is a free and Open Source telecom switch and billing system capable of providing and billing a range of telecom products and services to customers such as calling card products, residential and wholesale VoIP termination, DID resale and callback services.

A2Billing is commercially supported by Star2Billing S.L, where installation services, consultancy and support is available.

see http://www.star2billing.com/ for more details