3CX VoIP Phone System for Windows is an IP PBX / SIP proxy that completely replaces a traditional proprietary phone system. It uses standard SIP software or hardware phones, supports VoIP providers / SIP Trunks & phone lines and offers numerous benefits over a traditional PBX. The commercial editions offer enterprise grade support as well as a number of business features. A FREE edition is available here and is supported via the 3CX forums. A demo license key allowing you to try all commercial features for two simultaneous lines will be sent to your email address.