4PSA VoipNow is PBX software for service providers and enterprises. Easy to use and deploy, it can run on x86 or PowerPC based servers and delivers advanced IP telephony services. It can be deployed in various working scenarios to provide hosted VoIP services in high availability xSP environments, advanced call center services, or enterprise telephony in organizations of all sizes. A free Express version is available.
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.