What Are PRI and SIP Trunking?
PRI (Primary Rate Interface)
A physical T1 circuit delivering 23 voice channels to your PBX. Requires dedicated hardware (PRI card) and a physical connection from your telco.
- 23 channels per circuit
- Physical T1 line required
- Dedicated hardware card in PBX
- Fixed monthly cost regardless of usage
SIP Trunking
Voice calls delivered over your internet connection using the SIP protocol. No physical hardware needed beyond your existing network infrastructure.
- Unlimited channels (bandwidth-limited)
- Uses existing internet connection
- No special hardware needed
- Pay-per-use or per-channel pricing
Side-by-Side Comparison
| Feature | PRI | SIP Trunking |
|---|---|---|
| Channels | 23 per circuit (fixed) | Unlimited (scale on demand) |
| Monthly cost | $300-800/circuit | $0.01/min + $1.50/DID |
| Hardware needed | PRI card ($200-1000) | None |
| Installation | 2-8 weeks (telco dispatch) | Minutes (self-service) |
| Phone numbers | Sequential blocks from carrier | Any area code, any quantity |
| Encryption | Not available | TLS + SRTP |
| Failover | Requires second circuit | Automatic to backup server |
| Remote offices | Separate PRI per location | All locations on one trunk |
| Contract | 1-3 year typical | Month-to-month |
| Audio quality | Consistent (dedicated circuit) | Depends on internet quality |
Cost Comparison
The cost difference is where SIP trunking wins decisively for most businesses. Here is a real-world comparison for a company with 20 concurrent calls and 10,000 minutes/month:
PRI Cost (monthly)
- 1x PRI circuit (23 ch)$500
- DID block (20 numbers)$50
- Long distance (10k min)$200
- Hardware amortization$25
- Total$775/mo
SIP Trunk Cost (monthly)
- No circuit fee$0
- 20 DIDs @ $1.50$30
- 10k minutes @ $0.01$100
- No hardware$0
- Total$130/mo
83% savings: In this example, switching from PRI to SIP saves $645/month or $7,740/year. The savings increase with higher call volumes since SIP per-minute rates are typically lower than PRI long-distance charges.
Scalability
This is where the fundamental architectural difference matters most:
PRI Scaling Problems
Need 24 concurrent calls? You need a second PRI circuit ($500+/mo more), a second PRI card, and a 2-8 week installation window. Channels come in blocks of 23 - you cannot add just one more.
SIP Scaling
Need more concurrent calls? Nothing to change on the trunk side. Your only limit is internet bandwidth (one G.711 call uses ~87 kbps). A standard 100 Mbps connection can handle 1,000+ simultaneous calls.
Reliability Considerations
PRI's main advantage has traditionally been reliability - a dedicated circuit is not affected by internet congestion. However, modern SIP trunking has closed this gap:
| Scenario | PRI | SIP |
|---|---|---|
| Internet outage | Not affected | Calls fail (mitigated by failover) |
| Physical line cut | All calls down | Automatic failover to backup |
| Power outage | Both fail without UPS | Both fail without UPS |
| Provider outage | No failover option | Route to secondary provider |
| Disaster recovery | Requires new circuit install | Redirect to any location instantly |
Best Practice: For production SIP deployments, use a dedicated internet connection (or QoS-tagged VLAN) for voice traffic. This eliminates the "internet congestion" concern and gives you PRI-level audio quality with SIP flexibility.
Migrating from PRI to SIP
If you are currently on PRI and want to switch to SIP, here is the migration path:
- Set up your SIP trunk alongside your existing PRI (they can run in parallel)
- Get a test DID and verify inbound/outbound calls work
- Port your numbers from the PRI carrier to your SIP provider
- Update Asterisk routing to use the SIP trunk for outbound
- Remove the PRI card and cancel the circuit once fully migrated
Parallel Operation: You can run both PRI and SIP simultaneously during migration. Route outbound calls through SIP first, then port inbound DIDs one at a time. This gives you a risk-free migration path.
When PRI Still Makes Sense
PRI is not dead, but the use cases are narrowing. PRI may still be appropriate when:
- No reliable internet: Rural locations with poor broadband may still need PRI
- Regulatory requirements: Some industries require dedicated circuits for compliance
- Elevator/alarm lines: Some fire/safety systems still require POTS/PRI connectivity
- Existing long-term contracts: Breaking a PRI contract may cost more than finishing it
For the vast majority of businesses with reliable internet, SIP trunking is the clear choice in 2026.
The Verdict
For Asterisk and FreePBX users, SIP trunking wins on nearly every metric:
Average cost savings
Setup time vs weeks
Scalable channels
Replace Your PRI with SIP Trunking
IPComms makes the PRI-to-SIP migration painless. Run both in parallel, port your numbers, and cut your phone bill by 80%+.