Executive summary
A PRI (Primary Rate Interface) trunk is a physical E1 circuit in Mexico or T1 in the US that carries 30 simultaneous voice channels over copper. A SIP trunk is the IP equivalent: it uses the internet instead of a physical cable, and channel count is flexible based on the carrier contract.
Migrating from PRI to SIP trunk stopped being a trend in Mexico and is already the default for any new deployment. But PRI still has valid use cases — remote sites without reliable connectivity, legacy systems that don’t support SIP, or businesses with specific regulations requiring a dedicated circuit.
This guide covers:
- Real technical differences between PRI and SIP trunk
- Monthly cost comparison in Mexico (concrete scenarios)
- Scenarios where PRI is still the right choice
- Step-by-step migration plan without service interruptions
- Common risks and how to mitigate them
Primary CTA: Does your business still have PRI and you’re evaluating a migration? Quote your PBX → SIP migration and receive a SIP channels + DID portability proposal in 24h.
What changes technically when moving from PRI to SIP
PRI — the dedicated circuit
A PRI trunk is an ISDN interface defined in the 1980s to digitize the connection between the PBX and the carrier’s central office. In Mexico it typically runs over E1 (2.048 Mbps, 30 B channels at 64 kbps + 1 D signaling channel). In the US it uses T1 (1.544 Mbps, 23 B channels + 1 D channel).
Key characteristics:
- Physical: dedicated copper cable between your PBX and the carrier’s central office
- Fixed: 23 or 30 simultaneous channels, regardless of your actual traffic
- Cost: high fixed monthly fee plus per-minute cost to out-of-zone destinations
- Latency: very low and consistent (~1-3 ms in the last mile)
- Availability: high, but depends on local copper provider
SIP trunk — the IP equivalent
A SIP trunk uses the SIP protocol defined in RFC 3261 from the IETF [IETF RFC 3261, https://datatracker.ietf.org/doc/html/rfc3261, accessed 2026-09-06] to establish voice sessions over any IP connection. Cisco, in its official documentation, describes SIP as “a signaling protocol that controls multimedia sessions — voice, video, messaging — between endpoints” [Cisco, https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/configuration/cube-book/voi-understanding-sip.html, accessed 2026-09-06].
Key characteristics:
- Physical: none — you only need internet with QoS or voice VLAN
- Flexible: you pay for contracted channels, can scale up or down monthly
- Cost: monthly fee per channel + minutes at variable rates
- Latency: depends on your internet (ideally <50 ms to carrier)
- Availability: depends on internet redundancy, not physical copper
3CX summarizes it like this: “a SIP trunk can carry multiple simultaneous calls over an IP connection, without dedicated hardware” [3CX, https://www.3cx.com/learn/what-is-sip-trunk/, accessed 2026-09-06].
Cost comparison: PRI vs SIP trunk in Mexico
The numbers below are typical Mexican market ranges for 2026 — not specific quotes. For US ranges, Telnyx and Flowroute were used as reference [Telnyx pricing, https://telnyx.com/pricing; Flowroute pricing, https://www.flowroute.com/pricing, accessed 2026-09-06].
Scenario 1: business with 15 channels, average national traffic
| Concept | PRI (E1) | SIP trunk |
|---|---|---|
| Monthly fixed cost | MXN $3,500-5,500 (1 E1) | MXN $3,000-6,000 (15 channels) |
| National minutes included | 0 (pay-per-use) | Variable by plan |
| National per-minute | MXN $0.30-0.80 | MXN $0.05-0.15 |
| International per-minute | MXN $1.50-3.50 | MXN $0.50-1.50 |
| DID setup | n/a | MXN $250-500 |
| DID monthly | n/a | MXN $80-150 |
Read: for a business with 15 channels and moderate traffic (~5,000 min/month), monthly fixed cost is comparable. The significant difference is in per-minute cost — SIP trunk is typically 3-5x cheaper on national destinations.
Scenario 2: business with 30+ channels (call center, contact center)
At this scale, PRI implies 2 E1s (~$7,000-11,000 MXN/month just on trunks, before minutes). SIP trunk lets you adjust channels by season — Black Friday, Christmas, end of month — and pay only for what you use.
A typical business with 30 SIP channels pays MXN $6,000-12,000/month in channels, with the flexibility to scale up to 50 channels in a peak month and down to 20 in low season.
Scenario 3: business with high international traffic
Here’s where the difference is more marked. PRI charges international minutes at the local carrier rate — typically MXN $1.50-3.50/min. SIP trunk over a wholesale carrier can offer MXN $0.50-1.50/min depending on destination. A business that makes 10,000 international minutes per month can save MXN $10,000-20,000/month on this line item alone.
When PRI is still the right choice
SIP trunk isn’t universally better. There are three scenarios where PRI remains a valid option in 2026:
1. Remote sites without reliable IP connectivity
A plant in a rural area with high-latency satellite internet (>300 ms) or no carrier redundancy is not a candidate for SIP trunk. PRI over dedicated copper remains more stable in that context.
2. Legacy PBX that doesn’t support SIP
If your PBX is a proprietary system from the 90s/2000s (Avaya Definity, Nortel Meridian, Ericsson MD110) without the ability to add SIP, migrating to SIP requires replacing the entire PBX — a cost that may not be justified if operations are stable.
3. Regulations requiring a dedicated circuit
Certain financial or defense sectors have regulatory requirements for a dedicated encrypted line. PRI complies; SIP trunk over public internet does not — unless you use a certified SBC (Session Border Controller) and a dedicated VPN to the carrier.
PRI → SIP migration plan in 5 steps
Migrating without service interruption requires forethought. This is the sequence that minimizes operational risk:
Step 1 — Traffic audit (2 weeks before)
Before quoting, you need real data:
- CDR from the current PRI: total minutes, peak concurrent minutes, distribution by destination
- Number of active DIDs and which ones are critical (non-portable, customer-facing numbers, etc.)
- Extension inventory: how many IP phones, softphones, analog with ATA
Output: document with “peak channels = X, minutes/month = Y, main destinations = Z”.
Step 2 — Carrier selection (1 week)
Quote at least 2-3 carriers. The critical questions:
- How many channels do I need based on Step 1’s peak?
- Do they support my current PBX? (Asterisk, 3CX, FreePBX, Cisco, Avaya, etc.)
- What’s their IP-only auth or digest auth?
- Do they provide daily CDR?
- Do they have DIDs in my current prefixes?
Step 3 — DID portability (5 business days before the switch)
Mexico guarantees portability in 5 business days via NOM-189-SCT. Initiate the process with the new carrier at the same time you sign the contract. No cost if you keep the same geographic numbers.
Step 4 — Parallel configuration (1-2 weeks)
Configure the SIP trunk on the PBX in parallel with the existing PRI. This means:
- Add the SIP trunk as a secondary trunk
- Configure outbound routes (LCR — Least Cost Routing) so PRI remains primary initially
- Make test calls during low-traffic hours
- Validate CDR and billing
Step 5 — Cutover (1 day)
The day of the switch:
- Move PRI to backup trunk (only for emergencies)
- Activate SIP trunk as primary in LCR
- Monitor CDR in real time for 24h
- Keep PRI active for 30 days as backup before cancelling
Common risks and how to mitigate them
Risk 1 — Quality loss due to insufficient internet
Symptom: choppy audio, echo, dropped calls. Cause: insufficient bandwidth (each G.711 μ-law call requires ~80 kbps), high jitter to the carrier. Mitigation: dedicated internet contract with SLA ≥99.5% and jitter <30 ms. If your current provider can’t guarantee that, the SIP trunk will never sound right.
Risk 2 — Outages due to IP connectivity loss
Symptom: no inbound or outbound calls when internet drops. Cause: single dependency on one internet provider. Mitigation: dual internet path (two different ISPs with BGP or failover), or keep PRI as automatic backup for the first 6 months.
Risk 3 — STIR/SHAKEN in US for businesses with cross-border traffic
Symptom: calls to US are flagged as “spam likely” or rejected. Cause: carrier without signed A attestation. Mitigation: use a carrier with STIR/SHAKEN A attestation to the US [FCC 20-42, https://www.fcc.gov/document/stir-shaken-implementation-deadline-extensions, accessed 2026-09-06].
Risk 4 — Blockage by carrier SBC
Symptom: calls registered but no bidirectional audio. Cause: NAT, firewall or codec negotiated incompatibly. Mitigation: open SIP ports (5060/5061) and RTP range (10000-20000) on firewall; verify negotiated codec (G.711 μ-law typical).
Migration checklist
- PRI CDR analyzed (peak channels, minutes, destinations)
- 2-3 carriers contacted with written quotes
- PBX ↔ carrier compatibility confirmed
- DID portability initiated (NOM-189-SCT, 5 business days)
- Dedicated internet with jitter <30 ms
- SIP trunk configured in parallel with PRI as primary
- Test calls during low-traffic hours
- Failover plan documented
- Cutover executed with 24h monitoring
- PRI cancelled after 30 days of validation
When NOT to migrate yet
If your business meets any of these conditions, keep PRI:
- Site without reliable dedicated internet
- Legacy PBX requiring complete replacement to support SIP
- Minimal international traffic and competitive PRI rate
- Industry-specific regulations requiring a dedicated circuit
- Recent investment in PRI (< 2 years) with an active contract
In all other cases, SIP trunk is the technically and economically correct option for 2026.
Next steps
Migrating from PRI to SIP is a 2-4 week process if planned correctly. The most expensive mistake is doing it under operational pressure (e.g. PRI down, carrier delivering bad service) — always migrate with time on your side.
Primary CTA: Evaluating a PBX → SIP migration? Quote your migration here and a specialist reaches out with channel, portability and failover scenarios tailored to your traffic.