Is SIP just another word for VoIP?
Summary
No. VoIP is the whole practice of carrying calls over IP networks, and SIP is one standard inside it that sets calls up, changes them, and ends them. The audio itself travels separately, usually over RTP. Most buyers only run into the term when they connect an office PBX to a SIP trunk or try to reuse SIP desk phones with a new provider.
Who does what on a call
| Piece | Job | Defined in |
|---|---|---|
| SIP | Creates, modifies, and ends sessions such as internet phone calls | RFC 3261 |
| SDP | Describes the session: media type, codec, and similar details, carried inside SIP messages | RFC 8866, which replaced RFC 4566 |
| RTP | Carries real-time audio and video | RFC 3550 |
RFC 3261 is blunt about SIP's limits: it is not a vertically integrated communications system, it does not provide services, and it is meant to be combined with other protocols. What it does add is a registration function. Phones and apps tell the system where they currently are, and proxy servers use that to route a call to the user's current location. That is why one extension can ring a desk phone in the office and a laptop in a hotel.
Buying a SIP trunk
If you keep an on-premises PBX and replace its outside lines with an internet-based service, the PBX and the carrier talk to each other in SIP. The SIP Forum's SIPconnect recommendation, now at version 2.0 (ratified December 2016), is the industry's attempt to standardize direct peering between SIP-enabled IP PBXs and VoIP provider networks.
Trunks are sold in channels. 3CX says each call you make or receive uses one channel, and AWS says trunks are generally charged per channel, sized to the maximum number of calls you expect at once.
Bringing your own desk phones
Because SIP is shared across vendors, a desk phone can sometimes follow you to a new service, but the fine print is specific. Microsoft Teams SIP Gateway is a good example. It supports Skype for Business IP phones running standard SIP firmware, Cisco IP phones on multiplatform firmware, and SIP devices from vendors such as Poly, Yealink, and AudioCodes. Cisco phones on enterprise firmware must be converted first. Each user needs a phone number with PSTN calling enabled, the phones can only call users who have phone numbers, and presence isn't published, so presence-based routing is off the table.
Sources
- rfc-editor.org/rfc/rfc3261
- rfc-editor.org/rfc/rfc8866
- rfc-editor.org/rfc/rfc3550
- sipforum.org/technology/sipconnect
- 3cx.com/pbx/sip-trunking
- aws.amazon.com/what-is/sip-trunking
- learn.microsoft.com/en-us/microsoftteams/devices/sip-gateway-plan
Related on this site
- What is VoIP, and how is it different from a regular phone line?
- What does a business need in place to run an IP phone system?
- Is a cloud PBX any different from hosted VoIP or a cloud phone system?
- Should you own your phone system's PBX or let a cloud provider run it?
- How much do desk phones and headsets add to a business phone setup?
- Is the $10 Microsoft Teams Phone license enough to call regular phone numbers?
Written by Kaz (pen name). Figures are copied from providers' published pricing pages on the dates shown. We have not used these products. Corrections: contact form.