This is SIP trunking 101: what it is, how it works, and when it makes sense. SIP trunking replaces the phone lines behind your business phone system, not the system itself. Instead of copper lines or a PRI circuit coming into your building, your PBX connects to the phone network over your internet connection, through virtual "trunks" that carry your calls as data. Same phone system, same handsets, same phone numbers. Different, and usually cheaper, lines.
If you run a modern phone system and the bill for the lines behind it has started to look out of proportion, that is the problem our SIP trunking service exists to solve. The rest of this guide explains the moving parts so you can judge whether it fits.
What is SIP trunking?
SIP stands for Session Initiation Protocol. The standard that defines it, RFC 3261, describes it as a signaling protocol for creating, modifying, and terminating sessions with one or more participants. In plain terms, SIP is the part of a phone call that says "ring this number," "they answered," and "hang up." The voice itself travels alongside it as a stream of small data packets.
A SIP trunk is the connection between your phone system and a carrier that speaks SIP. The carrier connects your calls to the public telephone network, so the person you call can be on a mobile phone, a landline, or anything else. They never know the difference.
How SIP trunking works
- Your PBX registers with the carrier. Your IP-based phone system (3CX, Yealink, Cisco, Avaya and others) is configured with the trunk details and connects to the carrier over the internet.
- Calls go out as data. When someone dials out, the PBX sends the call over the trunk, and the carrier hands it to the telephone network.
- Calls come in the same way. Your numbers point at your trunks, so inbound calls arrive at your PBX, which routes them through your auto attendant, ring groups and extensions exactly as before.
Many deployments also put a session border controller at the edge of the network. It manages and secures call traffic between your system and the carrier.
Trunks, channels, and how many you need
The word "trunk" causes confusion, because what you are really buying is call capacity: the number of calls that can be up at the same moment. The rule is simple. You need as many channels as the most calls your business carries at once, not one per employee. A thirty-person office rarely has thirty people on the phone at the same time.
That is the clearest difference from a PRI. A T1 PRI circuit has 24 channels, 23 of which carry calls and one of which carries signaling, and it is sold in those fixed blocks. An office that needs twelve lines pays for twenty-three, and the thirteenth caller at the busiest hour means ordering another circuit. SIP trunk capacity is set from your real calling pattern and adjusted up or down as you need, without anyone visiting your building.
SIP trunking vs PRI and analog lines
| SIP trunks | PRI or analog lines | |
|---|---|---|
| How calls arrive | Over your internet connection | A dedicated circuit or copper pair into the building |
| Capacity | Sized to your busiest moment and changed as needed | Fixed blocks (a PRI carries 23 calls) or one call per analog line |
| Adding capacity | A configuration change | New circuits or lines, and an install visit |
| Multiple locations | Locations can share the same trunks | Lines are tied to each building |
| If the site goes down | Inbound calls can reroute to another location or to mobile phones | The circuit ends at your building, at the phone system that just went down |
What you need before you switch
- An IP-based phone system. If your PBX can register SIP trunks, it almost certainly works. If it predates IP, we will tell you during the assessment whether connecting it still makes sense or whether a hosted phone system would cost less.
- An internet connection that carries clean voice. Call quality depends on packet loss and jitter far more than raw speed. Our free test at voiptest.com tells you in about a minute whether your connection is ready.
- A firewall that leaves voice alone. A feature called SIP ALG, often on by default, rewrites voice signaling in transit and breaks it. 3CX advises using a device where it can be switched off. Our No Service troubleshooting guide covers this and other firewall problems.
- A 911 address for every location. Kari's Law requires that anyone can dial 911 directly from a business phone system, and RAY BAUM'S Act requires the call to carry a dispatchable location. Our SIP trunking supports multi-site E911, so a call from a branch office reports the branch, not headquarters. Our guide to Kari's Law and RAY BAUM'S Act has the detail.
The benefits, without the hype
- Lower line costs. You stop paying for PRI capacity you never use, and pay for the call capacity you actually carry.
- Capacity that follows the business. Seasonal peaks and new hires are a configuration change, not a circuit order.
- Built-in rerouting. Because your trunks live in the network rather than in a copper circuit at your building, inbound calls can go to another location or to mobile phones while your connection is down, and return to your system when it comes back.
- One set of trunks for several sites. A second office can share the first one's numbers and capacity.
- Nothing for staff to relearn. The phones and the system they know stay exactly where they are.
SIP trunking vs a hosted phone system
SIP trunking replaces your phone lines and keeps your phone system. A hosted phone system replaces the phone system too, so there is nothing left in your server closet. If your PBX is modern and you are happy with it, trunks are the smaller change. If it is aging or holding you back, hosted usually costs less over its life. We compare the two models in depth in on-premise PBX vs cloud hosted PBX.
How a move to SIP trunks works with Telnamix
- Assess the PBX and the network. We confirm your system can register SIP trunks and that your connection carries clean voice, before anything is ordered.
- Size the trunks from your real calling pattern.
- Port the numbers. We file the porting paperwork with your current carrier and set up E911 addresses for every location.
- Cut over on a schedule you choose, then test calls in and out on every number.
Questions to ask any SIP provider
- Is my phone system compatible, and will you confirm that before I sign?
- Do you price per channel of capacity or per user, and is there a long-term contract?
- How do you handle 911 for each of my locations?
- Where do my calls go if my internet connection fails?
- When I call support, do I reach someone who can see the trunk end to end, or a reseller who opens a ticket with somebody else?
Our answers: compatibility is checked up front, pricing is by call capacity with no long-term contract, multi-site E911 is supported, calls reroute automatically, and because Telnamix runs its own platform and carrier services, US-based engineers can see your trunks from end to end. If that sounds like what you need, see our SIP trunking plans for the phone system you already own, or contact our team for an assessment.