mptc is a Perl script to help you work out whether you're on the cheapest mobile phone network/tariff for your phoning habits. You feed it an itemised phone bill and tariff pricing information, and it will tell you how much that bill would have cost on different tariffs. So far it only supports UK network pricing systems, but could easily be extended and applied in other countries.
The REMWAVE Carrier Core platform is a stable platform to set up a voice over IP carrier (ITSP). It contains a complete billing, rating, and routing engine and is used by some of the largest ITSPs. The Carrier Core is free for deployment and contains a billing engine (based on Oracle express 10g edition, though larger editions are available), a SIP proxy and registrar, a customer care system, and templates to set up a customer self service portal.