Direct answer
Every claim is sourced below
Both connect a business phone system to the public telephone network. A PRI does it over a digital telephone circuit with a set number of call channels. A SIP trunk does it over an IP connection, where the number of calls is a limit the carrier sets. That one difference shapes what you would notice as a customer: which port on your PBX the service plugs into, how you add capacity, and what the service depends on to stay up.
PRI stands for Primary Rate Interface. An interface, in this sense, is a defined way for a carrier's circuit to plug into a phone system. Cisco's guide to configuring ISDN PRI gives the structure. On a T1 circuit, a PRI offers 23 B channels and one D channel. The B channels carry the calls, one call each, and the D channel carries the signaling that sets calls up and ends them. On an E1 circuit the figures are 30 B channels and one D.
Atlantech Online sells both services, and its comparison of SIP trunking and PRI describes the T1 version in the same terms. A PRI circuit there has 23 voice channels for 23 concurrent calls, and one data channel for call-related functions such as caller ID.
SIP trunking is a service delivered over an IP connection. Your PBX and the carrier exchange SIP messages to set each call up, and the audio travels between them as a separate stream of packets, in the sequence we trace in how SIP trunking works. The connection is not divided into fixed call channels ahead of time.
It is tempting to file one of these under old telephony and the other under digital. Both carry voice digitally, and both end at the same public network. The difference is in the handoff between the carrier and your PBX.
The differences side by side
| What differs | PRI | SIP trunk |
|---|---|---|
| What arrives at your site | A digital telephone circuit, such as a T1 | Service over an IP connection |
| Where it connects | A PRI port on the PBX | The PBX's SIP trunk interface, across your data network |
| How calls are signaled | On the D channel | In SIP messages |
| How capacity is counted | B channels on the circuit, 23 on a fully configured T1 | A simultaneous-call limit set by the carrier |
| What it shares | A circuit of its own | An IP connection your data may also use |
Suppose your office's busiest hour puts 26 outside calls in progress at once. One T1 PRI configured with 23 B channels has room for 23 of them and no channel for the other three. Covering them means more circuit capacity. A second full T1 PRI would add 23 channels to solve a shortfall of three, and what smaller increments a carrier will configure is a question for that carrier.
On a SIP trunk, the same shortfall is a change to a limit. Each call is a session the two ends agree on, and it holds no fixed time slot on a circuit. So a carrier that sells capacity by the call can set the trunk to 26. Atlantech's April 2023 post on its SIP trunks gives an example from one of its customers, a local government that adds capacity over its SIP trunks during weather events such as snowstorms and tropical storms. Atlantech's comparison draws the same contrast. It describes PRI capacity as bought in blocks, and SIP capacity as lines added for a month and removed when they are no longer needed.
A higher limit at the carrier is one of several ceilings, because your PBX has a maximum of its own and so does your connection. Twilio's trunking requirements use a planning figure of 100 kbps for each simultaneous call on the G.711 codec, with overhead included. By that allowance, 26 calls need 2.6 Mbps available for voice. The figure to order is also a sizing decision. It comes from your busy-hour traffic and a margin you choose, as we work through in how many simultaneous calls you need, and it can come out above the highest count you have seen.
A PBX connects to a carrier through whatever interface it was built or licensed with. A system with a PRI card expects a PRI, and a system with a SIP trunk interface expects SIP. Atlantech's business phone page says it can deliver service to a customer's own PBX by SIP, PRI or analog, and the 2023 post explains the split. SIP trunks go to customers that already run a VoIP PBX, and PRI and analog lines go to customers with a legacy PBX. In that arrangement the choice follows the equipment you have.
A gateway loosens that constraint. Sangoma describes its digital gateways as devices that convert between PRI connections and VoIP connections, so a legacy phone system can use a SIP trunk. The PBX keeps seeing a PRI on its own port, and the gateway speaks SIP to the carrier. The PBX's PRI ports still set how many calls it can pass, so a gateway changes the carrier connection without lifting that ceiling. Whether your model can take this route is the question in using SIP trunking with an existing phone system.
Some organizations keep both. Atlantech's comparison mentions educational institutions that move to unified communications and keep a few PRI lines for legacy intercom equipment and for some disaster recovery plans. The same comparison allows that keeping a working PRI can be the right choice for an organization that is not growing or changing its communications.
Neither name settles reliability. A PRI has a circuit to itself, and Atlantech lists as a PRI advantage that it "does not rely on data bandwidth to support voice calls." A SIP trunk shares an IP connection unless you give it one of its own. Calls can degrade when that connection is congested, and they stop when it is down. But the same comparison points out that a PRI customer loses phone service when part of the circuit's own infrastructure is damaged, and it names weather, vandalism and aging cable among the causes. On the question of which is more reliable, its own answer is "It depends." The useful comparison is between the specific circuit and the specific connection you would be relying on.
Neither name settles security either. Atlantech disputes the common belief that PRI is the more secure of the two, and notes that copper cabling can be intercepted. It also says that voice run over the public internet without adequate network security can be exposed. In both cases the protection comes from how the service is built and run.
Cost depends on the quote more than on the interface. Atlantech's phone page describes its pricing as tailored to the customer and prints no rate for either connection. A fair comparison therefore needs two quotes built on the same inputs: the same number of simultaneous calls, the same phone numbers and the same calling. On the SIP side, count what the trunk adds to what you already pay for. That may be extra capacity on your data connection, and a gateway if the PBX needs one.
How long a PRI will stay on offer also sits outside the name. Atlantech wrote in 2023 that PRI and analog lines "continue to dwindle," while its phone page still lists PRI delivery today. If you plan to keep a PRI-based PBX for years, ask the carrier how long it will supply and support the circuit.
International Telecommunication Union (ITU-T)
“4.1.2.2 At the 1544 kbit/s primary rate the interface structure is 23 B + D. 4.1.2.3 At the 2048 kbit/s primary rate the interface structure is 30 B + D. ... the 64 kbit/s capacity of the D-channel may or may not be used to provide an additional B-channel, depending on the situation; e.g., 24 B for a 1544 kbit/s interface.”
International Telecommunication Union (ITU-T)
“8.1 Provision of power. The provision of power to the NT via the user network interface using a separate pair of wires from those used for signal transmission, is optional. 8.2.1 Power Available from the TE. The power available (from the TE) at the NT shall be a minimum of 7 watts.”
International Telecommunication Union (ITU-T)
“Physical/electrical characteristics of hierarchical digital interfaces. Status: In force.”
International Telecommunication Union (ITU-T)
“ISDN user-network interface layer 3 specification for basic call control. Status: In force.”
IETF
“This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences.”
IETF
“RTP provides end-to-end network transport functions suitable for applications transmitting real-time data, such as audio, video or simulation data, over multicast or unicast network services. RTP does not address resource reservation and does not guarantee quality-of-service for real-time services.”
Office of the Federal Register (eCFR)
“An interconnected Voice over Internet Protocol (VoIP) service is a service that: (i) Enables real-time, two-way voice communications; (ii) Requires a broadband connection from the user's location; (iii) Requires internet protocol-compatible customer premises equipment (CPE); and (iv) Permits users generally to receive calls that originate on the public switched telephone network and to terminate calls to the public switched telephone network.”
Electronic Code of Federal Regulations (National Archives / GPO)
“(a) [Reserved] ... Each interconnected VoIP service provider shall specifically advise every subscriber, both new and existing, prominently and in plain language, of the circumstances under which E911 service may not be available ... broadband connection failure, loss of electrical power, and delays that may occur in making a dispatchable location available.”
Electronic Code of Federal Regulations (eCFR), Office of the Federal Register
“All telecommunications carriers shall transmit all 911 calls to a PSAP, to a designated statewide default answering point, or to an appropriate local emergency authority as set forth in Sec. 9.5.”
Electronic Code of Federal Regulations (National Archives / GPO)
“A person engaged in the business of installing, managing, or operating multi-line telephone systems may not install, manage, or operate for use in the United States such a system, unless such system is configured such that a user may directly initiate a call to 911 from any station equipped with dialing facilities, without dialing any additional digit, code, prefix, or post-fix, including any trunk-access code such as the digit 9.”
Federal Register / Federal Communications Commission
“The Direct Final Rule would repeal approximately 21 rule provisions and rule parts ... that plainly no longer serve the public interest because they have sunset by operation of law; govern an expired event; regulate an obsolete technology; are no longer used in practice by the FCC or licensees; or are otherwise duplicative, outdated, or unnecessary.”
Electronic Code of Federal Regulations (eCFR), Office of the Federal Register
“(3) Will result in a copper retirement, which is defined for purposes of this subpart as: (i) The removal or disabling of copper loops, subloops, or the feeder portion of such loops or subloops; or (ii) The replacement of such loops with fiber-to-the-home loops or fiber-to-the-curb loops.”
Federal Communications Commission, via the Federal Register
“the Commission adopted a Report and Order that reduces regulatory barriers and costs that hinder the transition from outdated legacy networks and services to next-generation, Internet Protocol (IP)-based infrastructure ... This rule is effective May 20, 2026.”
AT&T Inc., filed with the U.S. Securities and Exchange Commission
“The Company's consolidated financial outlook assumes sustained declines in service revenues within its Legacy segment as it makes progress against its objective of powering-down its energy-intensive copper-based network across the large majority of its footprint by the end of 2029 and upgrading customers to advanced connectivity services powered by 5G and fiber.”
CenturyLink (Lumen Technologies)
“CenturyLink Resale - Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI) service is grandfathered effective July 1, 2026 and is no longer available as a new service effective in all CenturyLink states. ... The basic service consists of 23 B Channels and a D Channel with a transmission rate of 1.544 Megabits per second (Mbps).”
AT&T (Georgia Technology Authority GTA Direct contract)
“Pricing provided for a flat-rate PRI includes: T1 local channel, voice/data interface, and 23 B Channels. Telephone numbers are charged separately based on quantity required. ... Long distance services are not included and will be charged separately. ... You can use one D channel to control multiple ISDN PRI lines, freeing the 24th channel on additional ISDN PRI circuits to increase capacity.”
SIP.US
“$24.95 per channel per/month ... $19.99 per channel per month ... Includes local and long distance calls to the US48 states and Canada (excluding NWT) at no additional charge. ... Excludes any local, state or federal taxes and/or fees.”
Twilio
“48 States (Zone 1): $0.0100 / min ... Local origination: $0.0034 / min ... Unlimited Call Concurrency: Included ... No contracts and no channel limits.”
Flowroute
“Outbound rate (US lower 48 & Canada) beginning at $0.00833/minute ... Domestic Inbound rate starting at $0.005/minute ... no limit on simultaneous calls.”