3CX Phone System is a software-based IP PBX which replaces traditional PBX hardware phones. It is based on the SIP standard and supports standard SIP software / hardware phones. It allows calls to be made via a VOIP service provider or via the regular PSTN. Easy to install and configure, it includes a web-based configuration interface, voice mail, auto attendant, call hunting and other features. Both commercial and a free edition are available.