Glossary
SIP
Session Initiation Protocol is the set of rules VoIP devices use to find each other, ring, answer, hold, transfer and hang up. If VoIP is the road, SIP is the traffic signalling.
SIP, or Session Initiation Protocol, is the signalling standard used by VoIP phones, apps and phone systems to set up, manage and end calls. SIP handles the invitation to talk, the ringing, the answer, holds, transfers and the hang-up. The voice itself travels in a separate media stream. Almost every business VoIP device and every SIP trunk uses SIP, which is why equipment from different makers can work together.
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What it means
A phone call has two parts: the conversation and the arrangements around it. SIP handles the arrangements. When you dial, your device sends a SIP invitation to the phone system. The system finds the destination, tells it to ring, and relays the answer back. Hold, transfer and hang-up are SIP messages too. The audio itself is carried by a companion protocol once SIP has agreed where it should flow.
SIP is an open standard. That is why a desk phone from one manufacturer, a softphone from another and a hosted phone system from a third can work together, and why a business can connect its own PBX to a carrier over a SIP trunk without proprietary hardware.
How it is used in a business phone system
- Each desk phone and app registers to the phone system using SIP credentials, which is how the system knows where to ring an extension.
- Transfers, call parking and conference calls are SIP operations carried out by the phone system on behalf of the device.
- A SIP trunk uses the same protocol to connect a customer's own PBX to a provider for inbound and outbound calls.
- Firewalls and routers sometimes interfere with SIP registration; this is checked at setup rather than discovered later.
How 3CTel uses it
3CTel devices and apps register to the hosted phone system with SIP, and the system uses SIP to ring, transfer and park calls between them. Customers who keep their own PBX connect it to 3CTel over a SIP trunk instead. In both cases a specialist configures the devices or the trunk, verifies registration through the customer's network, and tests calls on a temporary number before the real number is ported.
SIP Trunking
Carrier-grade SIP trunks for the PBX you already run — channels, DIDs and failover sized to your traffic.
Learn more: SIP TrunkingDevices
Phones and headsets that arrive ready to plug in.
Learn more: DevicesBusiness VoIP
SIP calling that replaces landlines — or feeds the PBX you already own.
Learn more: Business VoIPCommon questions about SIP
Is SIP the same as VoIP?
No, although the words are often used interchangeably. VoIP is the general idea of carrying voice as data over an internet connection. SIP is one specific protocol that sets up and controls those calls. Most business VoIP uses SIP for signalling, so in practice a SIP phone is a VoIP phone, but VoIP could in principle use other signalling.
Do I need to understand SIP to run a business phone system?
No. On a hosted phone system the provider handles SIP configuration and the customer sees extensions, menus and apps. The term matters mainly when buying desk phones, which should be SIP-compatible, or when connecting your own PBX to a provider, where a SIP trunk is the product being quoted.
Related pages.
SIP Trunking
Carrier-grade SIP trunks for the PBX you already run — channels, DIDs and failover sized to your traffic.
Learn more: SIP TrunkingBusiness VoIP
SIP calling that replaces landlines — or feeds the PBX you already own.
Learn more: Business VoIPDevices
Phones and headsets that arrive ready to plug in.
Learn more: DevicesNot sure which of these you need?
Describe how your calls arrive today and a 3CTel specialist will map them to the right pieces, build them on a temporary number and quote the result before anything is billed.