--- title: "SIP Trunking: Definition, Sizing, Pricing and When to Use It" description: "What a SIP trunk actually provides, how to size channels from real concurrency, US pricing models, and an honest comparison with a cloud phone system." url: https://skipcall.io/en/blog/sip-trunking language: en last_updated: 2026-09-02 --- [Ask Claude about this article](https://claude.ai/new?q=I'm%20reading%20%22SIP%20Trunking%3A%20What%20It%20Is%2C%20How%20to%20Size%20It%2C%20and%20When%20to%20Choose%20It%20Over%20Cloud%22%20\(https%3A%2F%2Fskipcall.io%2Fen%2Fblog%2Fsip-trunking\).%20Read%20the%20article%2C%20then%20answer%20my%20questions%20about%20it.) [Ask ChatGPT about this article](https://chatgpt.com/?q=I'm%20reading%20%22SIP%20Trunking%3A%20What%20It%20Is%2C%20How%20to%20Size%20It%2C%20and%20When%20to%20Choose%20It%20Over%20Cloud%22%20\(https%3A%2F%2Fskipcall.io%2Fen%2Fblog%2Fsip-trunking\).%20Read%20the%20article%2C%20then%20answer%20my%20questions%20about%20it.) LinkedIn X [Back to blog](https://skipcall.io/en/blog) VoIP Published 2 September 2026 # SIP Trunking: What It Is, How to Size It, and When to Choose It Over Cloud What a SIP trunk actually provides, how to size channels from real concurrency, US pricing models, the network prerequisites you now own, and an honest comparison with a cloud phone system. 15-30% of headcount on calls simultaneously at peak — the basis for sizing channels 100 kbps the bandwidth one active channel consumes, in each direction 2 the situations where a SIP trunk still beats a cloud phone system 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. 15-30% of headcount on calls at peak 100 kbps per active channel, in each direction $1-5 per DID per month, on top of the channels ## 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](https://skipcall.io/en/blog/business-voip-phone-service). 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:** 1. **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. 2. **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](https://skipcall.io/en/blog/business-phone-system). ## Deploying a SIP trunk 01 #### Measure real concurrency 1 day Concurrent-call history over three months, peak and 95th percentile. That number, and only that number, determines how many channels to order. 02 #### Verify IP-PBX compatibility 2-3 days 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. 03 #### Qualify and prepare the network 2-3 days Upstream bandwidth at peak, latency, jitter, packet loss. Then voice prioritization, SIP ALG off, voice VLAN if relevant. Before testing, not after. 04 #### Set security caps and E911 records 1 day 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. 05 #### Port the numbers and cut over 1-3 weeks Keep the old connection live through testing. Never cancel before the port completes — the full process is in our guide to [business number porting](https://skipcall.io/en/blog/business-phone-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. Author [Charles Baldet](https://skipcall.io/en/author/charles-baldet) [Charles Baldet on LinkedIn](https://www.linkedin.com/in/charlesbaldet/) CEO & Co-Founder, Skipcall Charles is the CEO and co-founder of Skipcall. A sales commando with over 10 years of experience in B2B SaaS and complex strategic accounts, he has closed major deals with Stellantis, SNCF, RATP and Natixis. A specialist in the PUCCKA and MEDDIC methodologies, Charles regularly teaches sales at HEC's incubator and the Sorbonne. He was ranked among Les Echos' top 10 business angels under 35 in 2020. He also co-founded Getalead (B2B sales agency) and Getlab (SalesTech studio). [See all their articles](https://skipcall.io/en/author/charles-baldet) FAQ ## Frequently asked questions **What is SIP trunking?** A SIP trunk is the connection linking your on-premise phone equipment — an IP-PBX, an Asterisk or FreePBX server, 3CX or similar — to the public telephone network, over the internet using the SIP protocol. It is the pipe that carries your outbound calls and brings inbound ones in. It replaced legacy PRI and analog trunk circuits, and inherits their logic of simultaneous channels. **What is the difference between a SIP trunk and a cloud phone system?** A SIP trunk provides connectivity only: the intelligence — auto attendants, queues, ring groups, voicemail — stays on your equipment, which you administer. A cloud phone system provides both, hosted by the provider and billed per user. SIP trunking makes sense when you already run an IP-PBX worth keeping; cloud makes sense when you are starting fresh or replacing end-of-life equipment. **How many channels do I need?** One channel equals one simultaneous call, inbound or outbound. Real concurrency usually lands between 15 and 30 percent of headcount: a 50-person company rarely runs more than 10 to 15 parallel calls. The right move is to read your current system's history rather than estimate — most companies over-provision by a factor of three. **How is SIP trunking priced in the US?** Two models. Per-channel with a bundled or metered minute plan, commonly $15 to $30 per channel per month with unlimited domestic calling, or lower per channel with metered minutes. And per-minute or 'unlimited concurrent' models where you pay for usage rather than capacity. Metered suits inbound-heavy operations; bundled suits outbound-heavy ones. DIDs are billed separately, typically $1 to $5 each per month. **What are the network prerequisites?** The same as any voice over IP: roughly 100 kbps per active channel in each direction, latency under 150 ms, jitter under 30 ms and packet loss under 1 percent. Add voice traffic prioritization on the router and disable SIP ALG, which breaks signaling on many consumer-grade routers. Bandwidth is almost never the limiting factor — consistency is. **Is SIP trunking still relevant in 2026?** Yes, in two specific situations: when you run a recent, amortized, well-understood IP-PBX that nothing justifies replacing, and when a regulatory or sovereignty constraint requires the call processing core to stay on your premises. Outside those two, a cloud phone system wins on total cost, mobility and service continuity. ## A calendar full of meetings starts with a call. [Get started now](https://app.skipcall.io/signup) [Book a demo](https://skipcall.io/en/demo) **No commitment** · Number live in 2 minutes Keep reading ## Related articles [View all articles](https://skipcall.io/en/blog) 01 VoIP ### [Business Phone System: How It Works, What It Costs, and How to Choose in 2026](https://skipcall.io/en/blog/business-phone-system) PBX, IP-PBX, or cloud phone system: the three generations compared, how call routing actually works, 2026 per-user pricing, and a decision grid by company size. 2 Sept 2026 02 VoIP ### [Business VoIP: Real Costs, Network Requirements, and a Rollout Checklist](https://skipcall.io/en/blog/business-voip-phone-service) How VoIP actually works, the network quality it really needs, a full cost breakdown, an eight-point rollout checklist, E911 obligations, and how to diagnose call quality problems. 2 Sept 2026 03 VoIP ### [POTS Line Retirement: What Every Business Needs to Do Before the Copper Sunset](https://skipcall.io/en/blog/pots-line-retirement-business) The real copper retirement timeline, the FCC rules that accelerated it, the forgotten devices that fail on cutover day, and a six-step migration plan that keeps the phones up. 2 Sept 2026 ## Ready to multiply your productivity? 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