If you're still paying for a stack of copper phone lines or a PRI circuit, there's a good chance you're overpaying for something that's on its way out. SIP trunking comes up in almost every phone system conversation I have, and it's usually explained badly. Here's what it actually is, who it's for, and how to tell whether it makes sense for your business.
A SIP trunk replaces traditional phone lines by delivering your calls over the internet to the phone system you already own. You probably need it if you have your own PBX (FreePBX, Avaya, Mitel, Cisco, NEC and so on). You probably don't if you're a small team without a phone system, or you're already on a hosted service like RingCentral or Teams Phone with a calling plan.
Think of your phone system as two parts: the PBX, the box or server that rings desk phones, runs the auto-attendant and routes calls to extensions, and the lines that connect it to the outside world.
For decades those lines were physical: individual analog copper lines, or a PRI circuit that carries 23 calls over one T1. A SIP trunk does the same job virtually. Your PBX connects to a carrier over your internet connection, and the carrier hands your calls to the public phone network. Your phone numbers, extensions and desk phones stay the same. Only the "pipe" changes.
SIP trunks are sold in channels. One channel is one simultaneous call, inbound or outbound. Ten channels means ten people can be on outside calls at the same time.
The key distinction most people miss: SIP trunking and hosted VoIP are not the same thing. With hosted VoIP, the whole phone system lives in the provider's cloud. With SIP trunking, you keep your phone system and just swap out the lines feeding it.
| Feature | Copper / PRI | SIP Trunking | Hosted VoIP |
|---|---|---|---|
| Who owns the phone system | You | You | The provider |
| Connection | Dedicated copper or T1 | Your internet | Your internet |
| Adding capacity | New circuit, often weeks | Add channels, often same day | Add a user license |
| Typical pricing | Per line or per circuit | Per channel or per minute | Per user, per month |
| Outage failover | Limited | Reroute to cell phones or another site | Built in |
| Long-term outlook | Being phased out by carriers | Current standard | Current standard |
Here's the quick checklist I walk clients through:
If you're weighing that last point, I break down the full cost comparison in On-Site vs. Hosted VoIP: Which Is Right for Your Business?
Not one per employee. Most offices never have everyone on an outside call at the same moment. A reasonable starting point:
Look at your call records for peak simultaneous calls on your busiest day, then add a small buffer. Many carriers also offer metered or burstable plans so you aren't paying for idle capacity.
SIP trunking is only as good as the network under it. Before switching, make sure you have:
Enough bandwidth. Plan on about 100 kbps in each direction per simultaneous call with standard G.711 audio. Twenty calls is roughly 2 Mbps up and down, which most business connections handle easily.
Quality of Service (QoS). Configure your firewall to prioritize voice traffic so a big upload doesn't make calls choppy.
SIP ALG turned off. This router "helper" feature is one of the most common causes of one-way audio and dropped calls. I cover it in Why Your Business VoIP Calls Keep Dropping.
A plan for outages. Set failover rules with your carrier so calls go to cell phones or another site if your internet goes down. A backup connection, like a second ISP or LTE, is cheap insurance.
Not sure if your internet is ready? Run the free VoIP speed test. It checks jitter, latency and packet loss, which matter more for call quality than raw speed.
911 addresses. Each number needs a correct registered address, and federal rules (Kari's Law and RAY BAUM'S Act) require multi-line systems to allow direct 911 dialing and send a dispatchable location. This needs to be set up correctly, not left at defaults.
Number porting takes time. Moving your existing numbers to a new carrier usually takes anywhere from several days to a few weeks. Keep your old service active until the port completes.
Fax machines, alarm panels and elevator phones. These often don't work reliably over SIP without adapters or a dedicated line. Inventory them before you cancel copper.
The cheapest carrier isn't always the best. Ask about call quality, redundancy, support hours, and how fast they respond when something breaks.
Pricing varies by carrier and contract, but the savings usually come from three places: you stop paying for idle lines, you drop circuit and equipment fees tied to copper or PRI, and long distance is typically included or billed per minute at very low rates. For businesses paying for a bank of individual lines or a PRI, moving to SIP commonly cuts the monthly phone bill substantially. The exact number depends on how many channels you actually need, which is why sizing matters.
If you own a phone system and it still runs on copper or a PRI, SIP trunking is almost always worth evaluating. You keep the system and phones you've already paid for, gain flexibility and failover, and usually lower your monthly cost. If you don't own a phone system, skip SIP trunking and look at hosted VoIP instead.
Send me your current phone bill and a rough headcount. I'll tell you how many channels you actually need, what you could save, and whether SIP trunking or hosted VoIP makes more sense. No sales pitch, no lock-in contracts.
Email Me →No. Your existing numbers can be ported to the SIP carrier. Plan for the port to take several days to a few weeks, and keep your current service active until it's done.
With a solid internet connection, QoS and failover routing set up, SIP trunking is very reliable. In many cases it's more resilient than copper, because calls can automatically reroute to cell phones or another office during an outage.
VoIP is the general technology of sending calls over the internet. SIP trunking is one way to use it: connecting your own phone system to a carrier. Hosted VoIP is another, where the provider runs the phone system for you.