3CX is a software-based private branch exchange (PBX) based on the SIP (Session Initiation Protocol) standard. It enables extensions to make calls via the public switched telephone network (PSTN) or via Voice over Internet Protocol (VoIP) services. 3CX for Windows, Linux, and Raspberry Pi is an IP business phone system that supports standard SIP soft/hard phones, VoIP services and traditional PSTN phone lines.