SIP (Session Initiation Protocol)
The signaling protocol that establishes, manages, and terminates voice and video calls over IP networks.
Session Initiation Protocol (SIP) is the application-layer signaling standard used to initiate, modify, and terminate real-time communication sessions — primarily voice and video calls — over IP networks. SIP handles the 'phone call' mechanics: dialing, ringing, answering, placing on hold, transferring, and hanging up. The actual audio is carried separately by RTP (Real-Time Transport Protocol).
SIP is to VoIP what HTTP is to the web: the underlying protocol that makes everything work. When a SIP phone 'registers' with a PBX or hosted platform, it uses SIP to announce its location. When a call comes in, a SIP INVITE message is sent to the phone; when the user answers, a SIP 200 OK is returned; and when the call ends, a SIP BYE message terminates the session.
Understanding SIP matters for businesses managing their own IP-PBX or SIP trunks. For businesses using a hosted Cloud PBX like Zonitel, SIP operates transparently in the background. All Zonitel devices and apps communicate using SIP over TLS (encrypted signaling), and media is secured with SRTP, protecting every call from interception.