SIP trunking is the least well understood piece of business telephony, because it is invisible. It appears on no screen, delivers no service directly to a user, and yet it determines how many calls your company can hold at once.
It is also the piece people buy by default, because “it is what we had before” — except that before, it was a PRI, and the comparison deserves to be made again. This guide explains what a SIP trunk actually is, how to size it, what it costs in the US, and when it still beats a cloud phone system.
What a SIP trunk actually is
A SIP trunk connects your on-premise phone equipment to the public telephone network, over the internet, using the SIP protocol.
The closest analogy is an electrical service drop: the trunk brings the current, it says nothing about what you do with it. Your auto attendants, queues, ring groups and voicemail all live on your equipment — a physical or software IP-PBX: Asterisk, FreePBX, 3CX, or a vendor appliance.
It replaced the legacy digital circuits:
| Legacy circuit | Simultaneous calls | Replaced by |
|---|---|---|
| Analog trunk lines | 1 per line | SIP trunk, one channel each |
| T1 / PRI | 23 | SIP trunk, 23 channels |
| Multiple PRIs | 23 per circuit | SIP trunk, channels as needed |
That continuity explains why SIP trunking is the default choice when carriers retire copper: it lets you keep the existing IP-PBX and change only the connection. That is a genuine advantage — provided the IP-PBX in question is worth keeping, which is the question nobody asks seriously.
The vocabulary
- Channel (also session or trunk line): one simultaneous call. Two channels means two conversations in parallel, in any direction.
- DID: the direct-inward-dial numbers attached to the trunk. DID count is independent of channel count — you can have 60 DIDs and 10 channels.
- SIP registration: how your IP-PBX authenticates to the provider, either by credentials or by authorized static IP.
- Codec: voice compression. Uncompressed G.711 gives the best quality at roughly 100 kbps per channel.
Sizing the channels
This is the only genuinely structural technical decision, and it should be made with numbers, not instinct.
The baseline
Real concurrency usually falls between 15 and 30 percent of headcount in a normal business. It goes higher only where the phone is the job: contact centers, high-volume support, calling floors.
| Profile | Headcount | Channels to plan |
|---|---|---|
| Normal business, occasional phone use | 20 | 4 to 6 |
| Business with reception and support | 50 | 10 to 15 |
| Multi-site, heavy phone use | 120 | 25 to 35 |
| Calling floor, phone is the job | 30 agents | 30 to 35 |
The method that beats estimating
Do not guess: your current system already knows. Pull the concurrent-call history for the last three months and take the peak, then the 95th percentile. Size on the 95th percentile, not the absolute peak — one annual spike does not justify paying twelve months for idle channels.
US pricing
Two billing models, suited to different usage.
Per channel, with bundled minutes
A monthly charge per channel including unlimited domestic calling, commonly $15 to $30 per channel per month. Clean and predictable, and the better fit for outbound-heavy operations where minutes accumulate.
Per channel, with metered minutes
A lower per-channel charge — often under $10 — plus per-minute usage. Better for inbound-heavy operations, where you pay for capacity rather than traffic.
DIDs are billed separately in both models, typically $1 to $5 per number per month, and toll-free inbound is almost always metered.
SIP trunking is nearly always cheaper on subscription than a cloud phone system. What costs money, and appears on no quote, is operating the IP-PBX it feeds.
What the quote leaves out
- DIDs beyond those included, and toll-free inbound minutes.
- International destinations, usually out of bundle — check line by line if you call abroad.
- Keeping the IP-PBX healthy: security patching, backups, in-house skill or a managed services contract. It is the heaviest line, and it never appears next to the trunk.
- E911 configuration per location, which is a legal obligation, not an option — see below.
Network prerequisites, now yours
Identical to any voice over IP, with one important difference: here you own them, not the cloud provider.
| Metric | Acceptable | Symptom beyond |
|---|---|---|
| Bandwidth | ~100 kbps per active channel, each direction | Choppy audio when all channels are busy |
| Latency | < 150 ms | Talking over each other |
| Jitter | < 30 ms | Metallic voice |
| Packet loss | < 1% | Missing words |
Three settings cover most incidents: voice traffic prioritization on the router, SIP ALG disabled — the feature meant to help SIP that breaks it on many consumer-grade routers — and a dedicated voice VLAN beyond roughly twenty seats. The diagnostic table is in our business VoIP guide.
Two obligations that are not optional when you own the PBX:
Cap outbound calling from day one. An IP-PBX exposed to the internet is a target. Set an allow-list of permitted international destinations, a maximum calls-per-hour per account, and a spend alert. Toll fraud means compromising a SIP account to place mass calls to premium-rate destinations, typically overnight or over a weekend. Uncapped, the bill runs into thousands over a single weekend.
Configure E911 per location. Kari’s Law requires direct 911 dialing without a prefix and an on-site notification when 911 is called; RAY BAUM’S Act requires a dispatchable location to be transmitted. With a cloud provider, much of this is handled for you. With your own PBX and a SIP trunk, it is your configuration, and it needs updating whenever a site or floor plan changes.
SIP trunk or cloud: the honest comparison
| Criterion | SIP trunk + IP-PBX | Cloud phone system |
|---|---|---|
| What you buy | Connectivity | Connectivity and intelligence |
| Billing | Per channel | Per user |
| Who administers | You, or your MSP | The provider |
| Adding a user | Configuration on the PBX | Minutes, self-service |
| Remote work and mobility | Possible, to configure and secure | Native |
| Security patching | Yours | Continuous, transparent |
| Site outage | Phones down | Calls fail over to mobile |
| E911 maintenance | Yours | Largely handled |
| Apparent cost | Lower | Higher |
| 5-year total cost | Often comparable, once operations are counted | Predictable |
SIP trunking remains the right call in two cases:
- You run a recent, amortized, well-understood IP-PBX, with custom development or line-of-business integrations that would be expensive to reproduce. Changing the connection without touching the core is then rational.
- A regulatory or sovereignty constraint requires call processing to stay on your premises.
In every other case, and particularly if your IP-PBX is nearing end of life or more than a third of your people work outside the office, the cloud system wins. The three-generation comparison is in our business phone system guide.
Deploying a SIP trunk
Measure real concurrency
Concurrent-call history over three months, peak and 95th percentile. That number, and only that number, determines how many channels to order.
Verify IP-PBX compatibility
Version, supported codecs, the registration method the provider accepts, NAT handling. An aging PBX can appear to work and produce subtle signaling problems for months.
Qualify and prepare the network
Upstream bandwidth at peak, latency, jitter, packet loss. Then voice prioritization, SIP ALG off, voice VLAN if relevant. Before testing, not after.
Set security caps and E911 records
International allow-list, hourly cap per account, spend alert. Plus a dispatchable location per site and floor. The highest-return hour of the whole project.
Port the numbers and cut over
Keep the old connection live through testing. Never cancel before the port completes — the full process is in our guide to business number porting.
What to take away
A SIP trunk is a pipe, not a phone solution. It does not replace a phone system: it feeds the one you already operate.
So the question to settle is not “SIP trunk or cloud”, but “do I want to keep operating an IP-PBX?”. If the answer is yes because the equipment is good and the expertise is in-house, SIP trunking is the modern, clean, economical connection. If the answer is “we never really thought about it”, then the question is worth asking before renewing, out of habit, an architecture designed for the PRI era.
