SIP trunk vs PRI: The real differences and why it matters now
As the copper networks that PRI depend on are retired around the world, now is the time to plan your migration to SIP trunking. Find out how the two technologies differ and what additional benefits your business can get with scalable, IP-based voice calling.
If the copper wire networks that support PRI are being retired in your country, then you will be looking at alternatives, and SIP trunking is the option that most businesses are going for.
There are good reasons for this.
PRI is a physical, circuit-switched connection over copper that carries up to 23 channels on a T1 or 30 on an E1. A SIP trunk is a virtual, packet-switched connection over IP, with capacity limited only by what your internet connection can support. Unlike PRI where you pay a fixed amount for the maximum capacity that you need, with SIP trunking capacity can be scaled up and down depending on requirements, so you don’t end up with infrastructure that is only fully utilized during peak periods.
In short, PRI is legacy ISDN telephony while SIP trunking is the all-IP standard replacing it. As they represent two generations of telephony, they are not two equal options.
In this guide we compare them side by side, looking at cost, switch-off timelines, and options for migration. We also look what additional benefits businesses can gain beyond a like-for-like swap.
What is PRI?
PRI stands for Primary Rate Interface. First introduced in the 1980’s, it has been the backbone of enterprise telephony for decades. It is a digital circuit that connects a PBX (private branch exchange, the system that routes calls inside your company) to the public telephone network, known as the PSTN.
PRI is circuit-switched. Each call holds a dedicated channel for its full duration. A T1 in North America provides 23 bearer channels plus one signaling channel. An E1, used in most other regions including Europe and APAC, provides 30 bearer channels. PRI.
What is SIP trunking?
SIP trunking is a virtual connection that carries voice, and other media, as data over IP between your phone system and the network. Channels are bounded by bandwidth, not by physical circuits. It is part of VoIP and underpins modern unified communications.
SIP trunks are not only for older phone systems. They also connect cloud PBXs, cloud contact centers and AI platforms. For the fundamentals, including how trunks are set up, read what SIP trunking is and why businesses need it.
See the grid below for a direct comparison of what PRI and SIP Trunks provide.
Are the two actually comparable?
Most comparisons that you will find online present PRI and SIP trunking as rivals and then admit SIP wins. The more accurate view is structural. PRI is a single interface within ISDN (Integrated Services Digital Network), built on circuit-switched TDM (time-division multiplexing). SIP trunking is the connectivity standard of the all-IP model that includes VoIP, programmable voice and unified communications.
In practice, you are comparing one legacy component with the category that replaces it.
Think of PRI as a multi-lane highway. To increase capacity, you have to widen it by building more road. For PRI that means ordering additional physical circuits and scheduling installs – all of which takes times. A SIP trunk is capacity you resize in software, on demand, without touching the cable in the ground.
SIP trunk vs PRI: A comparison
| Factor | PRI | SIP Trunk |
|---|---|---|
| Connection type | Physical copper, circuit-switched (TDM) | Virtual, packet-switched over IP |
| Concurrent channels | Fixed blocks of 23 (T1) or 30 (E1) | Bounded by internet connection |
| Scalability | Add circuits, often over weeks | Add channels in minutes |
| Setup time | Service visits and installation | Configuration and testing |
| Hardware | PRI cards or gateways, cabling | Minimal, an IP PBX or session border controller (SBC) |
| Media types | In most cases Voice only | Voice, video, messaging |
| Geographic flexibility | Tied to the site | Anywhere with IP connectivity |
| Disaster recovery | Limited failover | Rerouting to other sites or numbers |
| Call quality | Dedicated bandwidth | HD voice with quality of service (QoS) |
| Security | Physical isolation | Encryption such as SRTP, firewalls, SBC |
| Cost trend | Rising as copper is retired | Stable or falling |
| Global availability | Depends on local copper and ISDN, shrinking | Available wherever there is IP and a provider with local coverage |
| Future-proofing | Being phased out – already unavailable in some markets | The modern standard |
PRI and SIP trunk: Pros and cons
PRI
PRI earned its place. Its strengths are real, but so are its limits.
PROS
- Consistent quality: Dedicated bandwidth keeps call quality independent of internet load.
- Physical isolation: Calls are harder to intercept remotely.
- Traceable calls: Location details help with emergency calling.
- Predictability: It works well where bandwidth is limited.
CONS
- Rigid scaling: Capacity grows in blocks of 23 or 30 channels.
- Slow deployment: Installs often need service visits and weeks.
- Voice only: No video or messaging on the same connection.
- No native failover: Redundancy needs extra circuits.
- Rising costs: Maintenance gets pricier as carriers retire copper.
- Finite runway: It relies on infrastructure that is being switched off.
SIP Trunking
PROS
- Granular scaling: Add or remove channels as demand changes.
- Lower, more predictable cost: Savings are strongest on long-distance and international calls.
- Multimedia: Voice, video and messaging share one connection.
- Built-in resilience: Calls reroute if a site or path fails.
- Location independence: It suits hybrid and multi-site teams.
- Automation: Trunks can be provisioned through an API.
CONS
- Bandwidth dependence: You need enough capacity and QoS.
- Security setup: Firewalls, encryption and an SBC need proper configuration.
- Legacy equipment: Older analog hardware may need adapters or an IP PBX.
A quality carrier and a modern network setup mitigate all three of the SIP trunking cons.
Cost comparison: PRI vs SIP trunk
Total cost of ownership is where most buyers decide. Compare these five factors, not headline rates:
- Upfront hardware: PRI needs gateways or cards and cabling. SIP needs far less.
- Capacity model: PRI means paying for full 23 or 30 channel blocks. SIP lets you pay per channel, or scale elastically.
- Calling rates: SIP typically lowers long-distance and international charges.
- Maintenance: PRI support costs rise as copper is retired and legacy expertise gets scarce.
- Change costs: Moving or expanding PRI means new circuits. Changing a SIP trunk is a configuration task.
Industry estimates often put SIP at roughly 30 to 50 percent cheaper than PRI. However, your own savings depend on call volume, number of sites and destinations.
The global PSTN and ISDN switch-off
This is the reason the choice is effectively made. PRI is an ISDN service that runs over the copper PSTN, and carriers worldwide are retiring that infrastructure. PRI’s run is coming to an end, whatever your actual preference.
Why PRI is on borrowed time
ISDN, and therefore PRI, depends on the same aging copper the PSTN switch-off is decommissioning.
Carriers cite four drivers:
- Aging infrastructure: Copper networks are fragile and costly to maintain.
- Shrinking expertise: Fewer engineers know the legacy equipment.
- Declining usage: Fewer customers use traditional lines each year.
- Inefficiency: Circuit switching wastes capacity that packet switching uses fully.
A deadline that varies by country
Most coverage treats the switch-off as a US or UK story. In reality it is a global migration with an uneven timeline.
- Already complete or largely complete: Germany, Sweden, Singapore, Estonia, the Netherlands, and Spain.
- United Kingdom: BT and Openreach have set a final date of 31 January 2027, with stop-sell in place since September 2023.
- France: Orange is targeting around 2030.
- Australia: Migration is happening through the NBN.
- United States: It varies by region, with no single national date – although a non-obligatory date of 31 December 2028 has been proposed by the FCC. All providers have started the phase out, although Verizon have confirmed they will keep a minority of lines for rural areas that have no alternative at this point.
If you operate in several countries, plan market by market. Dates, porting rules and emergency-calling obligations differ in each.
When does PRI still make sense?
There are cases where keeping PRI for now is reasonable:
- Poor connectivity: A site with unreliable internet and no near-term fiber.
- Small, stable footprint: A handful of lines where change is not worth the effort before the local switch-off.
- Isolated legacy lines: Disaster recovery or niche systems that rarely change.
- Phased migrations: PRI runs in parallel behind a gateway while you move traffic over.
These are shrinking, transitional cases, not a long-term strategy. Treat them as a bridge with an end date.
How to migrate from PRI to SIP
Most businesses do not need to discard a working PBX. You can modernize without replacing it.
Connect your existing PBX with SIP trunking
SIP trunking connects an existing PBX or SBC to an IP network, so migration can be a planned step instead of a complete replacement. A SIP-to-PRI gateway, a device that translates between IP-based SIP and legacy PRI channels, lets you run old and new side-by-side during cutover.
Infobip SIP trunking works with existing infrastructure and platforms, including Freshworks, Genesys PureCloud, Cisco Webex and OpenAI Realtime. Trunks can be created, updated and deleted through an API. For many businesses, connecting an existing system to the Infobip network can take days, not months.
A practical migration checklist
- Audit what you have: Lines, channels and any systems that depend on the PSTN, such as alarms, lifts, fax and payment terminals.
- Check your network: Confirm bandwidth, QoS and SD-WAN readiness.
- Choose a provider: Look for the reach, local coverage, future footprint and outbound calling strategy.
- Port your numbers: This may not always be possible. Where you can, move existing DIDs (direct inward dialing numbers) and plan how numbers are handled in each country.
- Run a phased cutover: Keep a gateway in parallel while traffic moves.
- Validate emergency calling and failover: Test location data and backup routes before you rely on them.
- Decommission PRI: Cancel circuits once everything is stable.
Porting rules, local presence requirements and emergency-calling obligations differ by country. Ask each provider how they handle them in every market you operate in. That is where a provider with global coverage matters.
Beyond SIP trunking: where modern voice is heading
SIP trunking is the connectivity layer, not the destination. Moving off PRI is a chance to gain capabilities that copper was never able to support.
From connectivity to programmable and AI-powered calls
Once voice runs over IP, it becomes programmable for example:
- Calls API: Custom call control for your own voice applications.
- IVR: Interactive voice response flows that automate routine calls.
- Voice AI agents: Natural-language conversations that replace rigid menus and hand off to live agents with context. Voice AI agents are currently in Early Access, so confirm availability for your markets.
If you already use an external AI platform for calls, Authenticated SIP Trunking is the preferred connection. It adds carrier-grade, authenticated SIP over TLS and SRTP for external Voice AI providers, cloud contact centers and existing telephony, without replacing them.
Voice as one channel in an omnichannel platform
Voice does not need to live in a silo. On Infobip’s AgentOS platform, Voice shares customer profiles, journey orchestration and analytics with SMS, WhatsApp, RCS and chat apps. A call and a message belong to the same customer view, and a customer who called yesterday is recognized when they message today.
Voice is also available through APIs, so technical teams can integrate it without adopting the rest of AgentOS.
Modernize your voice with Infobip
Infobip Voice is a global calling platform. Its SIP trunking connects your existing PBX or SBC to Infobip’s network, which supports outbound calling to 255 destinations and inbound calls in 68 countries, across 40+ data center locations on four continents. Redundant routing and built-in retry logic support reliability at scale.
SIP trunking is one layer of Infobip Voice, alongside the Calls API, IVR and Voice AI agents.
Start with the SIP trunking documentation, or talk to the Infobip team about planning your move from PRI.
Frequently asked questions
A PRI is a physical, circuit-switched connection over copper, carrying 23 channels on a T1 or 30 on an E1, that links a PBX to the PSTN. A SIP trunk is a virtual, packet-switched connection that carries calls as data over IP, with channels limited by bandwidth. PRI is legacy TDM telephony, and SIP trunking is the all-IP standard replacing it.
In most cases, yes, especially at scale and across multiple sites. SIP avoids per-circuit hardware, scales per channel instead of in fixed blocks, and lowers long-distance and international rates. PRI costs are also rising as carriers retire copper.
It is a device that bridges IP-based SIP trunks to legacy PRI interfaces, translating signaling and media into PRI channels. It lets you modernize without immediately replacing an existing PBX. It is commonly used to run old and new systems in parallel during a phased migration.
Yes. PRI is an ISDN service that runs over the copper PSTN, and carriers worldwide are retiring that infrastructure. Several countries have completed the switch-off, the UK has a final date of 31 January 2027, and others follow regional timelines.
A PRI T1 provides 23 voice channels in North America, and an E1 provides 30 in most other regions, fixed per circuit. A SIP trunk can support far more concurrent calls, bounded by bandwidth, and channels can be added or removed in software.
Usually, yes. SIP trunking connects an existing PBX or SBC to an IP network, and a SIP-to-PRI gateway can bridge older equipment during the transition. You can modernize on your own timeline without a full rip and replace.
With adequate bandwidth, QoS and a quality carrier, SIP call quality is effectively indistinguishable from PRI, and SIP adds built-in failover. Security is a configuration question, not an inherent weakness. Firewalls, encryption such as SRTP and an SBC protect SIP traffic, and a private carrier network reduces exposure to the public internet.