What is the difference between SIP trunking and VoIP?

The NIST glossary takes its definition of VoIP from CNSSI 4009, a federal glossary. It reads "a group of technologies used to provide voice/video/collaboration communications over a data network using internet protocols." The definition names no product and no single protocol. An IP desk phone calling a colleague through an IP PBX fits it, and so does a call one carrier hands to another over IP. We go further into the category in what VoIP is.

SIP sits one level down, as a protocol used inside that group of technologies. RFC 3261, the standard that defines it, describes a protocol that can establish, modify and terminate sessions such as internet telephone calls. The same document says "SIP does not provide services." Somebody has to build a service from it.

SIP trunking is one such service. Twilio's API reference describes its trunking product as connectivity that lets IP-based communications infrastructure reach the PSTN, the public switched telephone network. A hosted phone service is another service built with the same technology. When someone offers you "VoIP" as an alternative to SIP trunks, the word has to mean a hosted service for the comparison to make sense.

A SIP trunk carries calls between your phone system and a carrier across an IP network. SIP messages manage each call and a separate stream of packets carries the audio, as we show in how SIP trunking works. That puts the trunk inside the NIST definition.

The category reaches well past trunks, though. A call between two IP phones on an IP PBX is VoIP, and the PBX can complete it without sending anything to the carrier. VoIP does not have to use SIP, either. RFC 3435 defines MGCP, a different protocol in which a call agent controls the gateways that turn telephone-circuit audio into packets. And VoIP does not have to cross the public internet. NIST's definition says "a data network," and Twilio's trunking documentation lists a private interconnect beside the public internet as a way to connect.

So "VoIP" on a quote or a spec sheet tells you calls travel over IP. It does not tell you which service you are being offered or who operates it.

Atlantech Online's business phone page puts the two purchases side by side. You can connect to its cloud PBX, or, if you have your own PBX, it can deliver service by SIP, PRI or analog. The cloud PBX is VoIP, and so is the own-PBX route when the delivery is SIP. They differ in where the phone system lives.

On the first route, Atlantech runs the phone system. Its UCaaS page says the hosted phone system is in Atlantech's cloud in place of a PBX in your office. The phone page lists auto attendants, call distribution groups, a receptionist console and voicemail transcription, and says an administrator can add users, devices, phone numbers and call queues. It also describes unlimited nationwide calling, so public calling is part of the package. A buyer who asks for "VoIP" and means "a phone system I don't have to run" is describing this route.

On the second route, you run the PBX and Atlantech connects it. Its April 2023 post on SIP trunks says SIP trunks are for customers that already have their own VoIP PBX. The dial plan, the voicemail and the call routing stay in a system you control, and you take on its upkeep. That is the trade in SIP trunking versus cloud PBX.

Neither route is as tidy as its label suggests. A hosted service still leaves equipment at your site. People need phones or apps, and the office network has to carry the calls. Atlantech lists headsets, handsets it supplies and customer-owned devices that work with its voice switch. A trunk, for its part, does not require a PBX in your building. Twilio's requirements name an IP PBX, a SIP call server and a session border controller as examples of what a trunk can connect to. A system of that kind can run on a server somewhere else, provided it can reach the carrier.

Suppose a 40-person office runs its own IP PBX with IP desk phones. Each phone connects to the PBX over the office network. When one employee calls another's extension, the PBX connects them itself, and that call is VoIP from end to end. When someone dials a customer's mobile, the PBX sends the call out through its SIP trunk, and the carrier passes it to the mobile network. VoIP is in use on both sides of the PBX. SIP trunking describes only the side that faces the carrier.

Now suppose the same office moves to a hosted service that bundles public calling, as Atlantech's does. The desk phones register with the provider's system over the office's data connection. A call between two extensions is set up by that system, and a call to the customer's mobile leaves from the provider's platform. In this arrangement the office has no PBX of its own and no separate trunk to order, because public calling comes with the service. Its calls are still VoIP.

The unit of the price follows from the service. A trunk is priced by the connection's capacity or use. SIP.US, which publishes its rates, charges $24.95 a month for each channel on its month-to-month plan, and a channel is one call in progress. Suppose the 40-person office decides to buy eight channels. They come to $199.60 a month before taxes, fees and phone numbers. Headcount does not enter into the sum. Eight is an assumed order here. The right count comes from sizing the trunk for your busiest hour.

A hosted service is priced for the phone system as well as the calling, and Atlantech's is priced by the user. Its UCaaS page advertises a Business Line starting at $12 per user per month and a Premium Line starting at $18. Those are starting rates, and the page prints no contract term, taxes or equipment charges beside them. At the Business Line rate, 40 users would come to $480 a month before any other charges, if every user is on that line and the quote comes in at the starting rate. So the same office counts 40 users on one route and eight calls on the other. Atlantech's phone page describes the pricing for its trunk route as tailored and prints no rate. We compare more per-user rates in what UCaaS costs per user.

The two totals are not comparable as they stand, because the trunk price buys only the connection. The PBX behind it has its own costs, in licenses, hardware or hosting, and in the time someone spends looking after it. Put those beside the trunk before you set it against a hosted quote.