An IP phone number is part of a Voice over Internet Protocol (VoIP) system, allowing communication over the internet rather than traditional phone lines. This type of phone number is SIP-based (Session Initiation Protocol), a signaling protocol used to initiate, maintain, and terminate real-time sessions like voice, video, and messaging applications. SIP is the backbone of IP telephony, providing the foundation for modern business communication.

When someone makes a call using an IP phone number, the SIP protocol manages the connection between the devices. Unlike traditional phone lines, SIP uses the internet to transfer data packets. The system first checks the availability of the recipient, then establishes the communication session, handles the exchange of audio or video, and finally ends the call when needed. The entire process is seamless and occurs in real time.

The core of this system revolves around SIP servers and SIP endpoints. SIP endpoints can be IP phones, softphones (software-based phones), or any device capable of handling SIP communications. The SIP server acts as a switchboard, routing calls to their destination.