SIP Trunking vs PRI: Why T1 Lines Are Dead

If you are still running PRI lines in 2026, you are operating on infrastructure that every major carrier in the United States has announced plans to decommission. AT&T began migrating customers off ISDN in 2020. Verizon and Lumen have published similar sunset timelines. The copper network that PRI depends on is being retired in favor of IP-based infrastructure, and the carriers maintaining it are raising prices to encourage migration. Staying on PRI is not conservative — it is expensive, limiting, and increasingly risky as the support infrastructure around it shrinks.

This guide compares SIP trunking and PRI from the carrier perspective. SIPNEX is an FCC-licensed carrier that provides SIP trunks for call centers, dialer operations, and businesses migrating off PRI. We have helped hundreds of operations make this transition, and the story is always the same: the operator wishes they had moved sooner.

What PRI is and how it works

PRI — Primary Rate Interface — is a telecommunications interface standard that delivers voice channels over a physical T1 circuit using ISDN (Integrated Services Digital Network) signaling. A single PRI carries exactly 23 Bearer channels (for voice) and one D channel (for signaling), totaling 24 channels on a 1.544 Mbps T1 line.

Each Bearer channel carries one phone call at 64 kbps using G.711 codec — the same codec used on SIP trunks. The D channel carries ISDN signaling messages (call setup, teardown, caller ID delivery, DTMF) at 64 kbps. The total bandwidth is fixed at 1.544 Mbps whether you are using all 23 channels or none of them.

PRI requires a physical T1 circuit delivered to your premises — typically over copper pairs from the local exchange carrier. Your PBX connects to the T1 via a PRI card (a hardware interface that speaks the ISDN protocol). The carrier’s central office switch handles call routing, number translation, and PSTN interconnection.

The PRI model was designed for a world of fixed offices, predictable call volumes, and carrier-managed infrastructure. It worked well for decades. It does not work well for modern business communications, and it is actively being retired.

The comparison: PRI vs SIP trunking

Channel capacity. PRI: exactly 23 channels per T1. Need 24 simultaneous calls? Buy a second PRI. Need 100? Buy five PRIs. The capacity is rigid, physical, and takes weeks to change. SIP: unlimited channels on carriers like SIPNEX. Your capacity is your bandwidth and your server resources. Need 10 channels today and 500 tomorrow for a campaign launch? Scale instantly. No hardware order, no installation appointment, no waiting.

Cost structure. PRI: $300 to $500 per month per T1, regardless of usage. You pay the same whether you use all 23 channels 24 hours a day or make five calls a month. Installation fees range from $500 to $2,000. PRI cards for your PBX cost $200 to $800. SIP: per-minute pricing based on actual usage. At SIPNEX wholesale rates, $400 (the cost of a single PRI) buys 25,000 to 80,000 minutes depending on your volume tier. No installation fee. No hardware purchase. The SIP trunk connects to your PBX via software configuration.

Geographic flexibility. PRI: the T1 terminates at a physical address. Your PBX must be at that address. If your office moves, the PRI moves with you (after a new circuit installation that takes weeks). If your office loses power or internet, the PRI goes down (though it may continue working briefly on battery backup since PRI does not depend on internet). SIP: the trunk terminates at an IP address. Register your PBX from your office, a data center, your home, or all three. Configure failover so incoming calls route to a backup location automatically when the primary goes down. Agents can register softphones from anywhere.

Features. PRI: caller ID delivery, basic call routing, DTMF. That is essentially the feature set. PRI does not support STIR/SHAKEN attestation, SMS, programmable routing, carrier-level call recording, or any feature developed after the 1990s. SIP: full STIR/SHAKEN attestation (A-level on SIPNEX), CNAM management, SMS/MMS on the same trunk, carrier-level failover, real-time CDR streaming, programmable call routing, and a feature set that continues to expand because SIP is software-defined.

Phone numbers. PRI: your DIDs are tied to the PRI circuit and the carrier that provides it. Porting numbers off PRI follows the standard LNP process but can be complicated by legacy provisioning systems. Adding new DIDs requires coordination with the PRI carrier and may take days. SIP: DIDs are provisioned instantly from the carrier’s nationwide inventory. Add local numbers in any area code, toll-free numbers, and vanity numbers through a portal or API. Port existing numbers from PRI to SIP through standard LNP (7-14 business days).

Redundancy and disaster recovery. PRI: failover requires a second physical PRI circuit at a second location, with your PBX configured for automatic failover between circuits. The second circuit costs the same as the first. If the local exchange carrier’s central office has an outage, both your primary and backup PRIs may be affected. SIP: carrier-level failover to any IP endpoint. Configure primary and secondary (and tertiary) registration endpoints. If your primary location goes down, the carrier reroutes incoming calls to your backup within seconds. The cost of SIP failover is configuration time, not additional circuits.

Compliance and attestation. PRI: no STIR/SHAKEN capability. Calls originated over PRI enter the telephone network as unsigned, which increasingly means they receive C-level attestation or no attestation at all when they are converted to SIP at the carrier’s gateway. This is a growing problem — as the network moves to SIP, PRI-originated calls look like gateway traffic and receive the lowest trust treatment. SIP: direct STIR/SHAKEN signing at the originating carrier. On SIPNEX, your calls receive A-level attestation because we sign them directly with our SP-KI certificate. Your calls enter the network with the highest trust signal available.

The PRI sunset timeline

This is not speculation. The carriers have published their intentions.

AT&T began decommissioning legacy TDM (Time Division Multiplexing) services including ISDN PRI in 2020. They have been migrating customers to IP-based alternatives and have raised prices on legacy services to incentivize migration. New PRI installations are unavailable in many AT&T service areas.

Verizon has been transitioning away from copper-based services for years, offering Fios (fiber) as the replacement for both consumer and business customers. PRI availability is declining as Verizon retires copper plant.

Lumen (formerly CenturyLink) has been shifting to IP-based voice services and has raised PRI pricing significantly. Their enterprise focus is on SIP trunking and SD-WAN, not legacy ISDN.

Regional carriers are following the same trajectory. The FCC has approved multiple carrier applications to retire legacy copper infrastructure. The regulatory framework that protected PRI customers by requiring carriers to maintain the copper network is eroding as the FCC acknowledges the transition to IP.

The practical implication: even if your current PRI works fine today, the support ecosystem around it is shrinking. Finding replacement PRI cards for your PBX is getting harder. Finding technicians who understand ISDN signaling is getting harder. Getting timely repairs on aging T1 circuits is getting harder. And the cost is going up, not down, because the carriers want you off the platform.

How to migrate from PRI to SIP

The migration is less disruptive than most operators expect. Here is the process.

Step 1: Inventory your current PRI setup. How many PRIs do you have? How many concurrent channels do you typically use (check your PBX CDRs for peak concurrency)? What DIDs are provisioned? What is your monthly cost (circuit charges + DID charges + per-minute charges if applicable)?

Step 2: Choose a SIP carrier. Evaluate based on the criteria that matter for your operation: attestation level, concurrent channel policy, pricing, codec support, and support depth. SIPNEX provides unlimited channels, A-level attestation, and published wholesale rates.

Step 3: Configure your PBX for SIP. If your PBX supports SIP (virtually all modern PBXes do — Cisco, Avaya, Mitel, Asterisk, FreePBX, VICIdial), add a SIP trunk configuration with the new carrier’s credentials. You may need to update your outbound number format to E.164 (+1XXXXXXXXXX) since PRI used 10-digit or 7-digit dialing conventions.

Step 4: Test with new DIDs. Before porting your existing numbers, provision new DIDs from your SIP carrier and test inbound and outbound calling, audio quality, DTMF, fax (if applicable), and caller ID display. This allows you to validate the SIP trunk without disrupting your current PRI service.

Step 5: Port your numbers. Submit LOAs (Letters of Authorization) to port your existing DIDs from the PRI carrier to your SIP carrier. Porting takes 7 to 14 business days for simple ports. During the port, your numbers continue to work on the PRI. On the FOC (Firm Order Commitment) date, the numbers cut over to SIP.

Step 6: Decommission PRI. Once all numbers are ported and the SIP trunk is handling all traffic, cancel your PRI circuits. Verify that the PRI carrier has released all numbers and that billing stops.

The entire process typically takes 3 to 4 weeks including porting. The SIP trunk can be configured and tested in a single day. The risk is minimal because you run both systems in parallel during the transition.

Frequently asked questions

SIPNEX provides SIP trunks that replace PRI with unlimited channels, A-level STIR/SHAKEN attestation, and wholesale pricing that makes PRI look like an expensive relic. Start your PRI migration or see our rates.