What Is SIP Trunking? A Plain-English Business Guide

2026-06-11 · 8 min read · by the fvoip team

If you own a business phone system (a PBX in a closet, wired to desk phones around the office), sooner or later a vendor will pitch you SIP trunking. The pitch usually arrives wrapped in acronyms. Here is the whole concept in one sentence: SIP trunking replaces the physical phone lines coming into your phone system with a connection over the internet. Your PBX stays. Your desk phones stay. The copper or PRI circuit from the phone company goes away, and calls travel over your internet connection instead.

What SIP trunking replaces

Traditionally, a business PBX connected to the outside world in one of two ways. Small offices used a handful of analog lines, one physical pair of wires per simultaneous call. Larger offices used a PRI (Primary Rate Interface), a digital T1 circuit that carries 23 simultaneous calls plus a signaling channel: 23B+D in telecom shorthand. PRIs were the workhorse of business telephony for decades, and they came with workhorse pricing, commonly several hundred dollars per month per circuit, plus per-minute long distance, plus multi-year terms.

The rigid part was the increments. Need 25 simultaneous calls? That is two PRIs, 46 channels, and you pay for all of them. Need to add capacity for a seasonal rush? That is a carrier order and a wait, then a contract that continues long after the rush ends.

How SIP trunking works instead

SIP (Session Initiation Protocol) is the standard language of internet calling; it is how phones and systems set up, connect, and end calls. A SIP trunk is a virtual version of those incoming circuits: a registered connection between your PBX and a carrier, delivered over broadband. Three terms cover most of what you will hear:

Channels (or call paths): the number of simultaneous calls the trunk carries. This is the direct replacement for physical lines. Unlike a PRI, channels come in increments of one, and adding one is a configuration change, not a construction order.
DIDs (Direct Inward Dial numbers): your phone numbers. Numbers and channels are separate: you can have 50 numbers riding on 10 channels if you never have more than 10 calls at once. Your existing numbers port over; the process is the same one described in our porting guide.
Concurrency: how many calls actually happen at once, which is what you size channels to. A common starting ratio for general office use is one channel per 3 to 5 employees, then adjust from real usage. A 40-person office rarely needs 40 channels; it might comfortably run on 10 to 12. Call-heavy operations (dispatch, appointment desks, sales floors) size higher.

Why businesses switch to SIP trunks

Cost. SIP channels typically run in the range of $10 to $25 per channel per month across the industry, usually with unlimited or generously pooled US calling, versus several hundred dollars for a 23-channel PRI plus usage. Most PBX owners cut their carrier bill dramatically on this move alone.
Right-sizing. Buy 12 channels because you need 12, not 23 because that is what a T1 holds.
Consolidation. Multi-site businesses can pool trunks centrally instead of paying for separate circuits at every location.
Failover. A physical circuit that goes down takes your calls with it. SIP trunks can be configured to fail over: if your PBX or internet is unreachable, calls redirect to backup numbers or mobile phones instead of ringing into a dead line.
Longevity of your PBX investment. If your PBX is modern and paid for, trunking lets you keep using it while shedding legacy carrier pricing.

What your PBX needs

Most PBX systems sold in roughly the last 15 years speak SIP natively, including later Avaya IP Office, Mitel, NEC SV-series, Panasonic NS-series, Grandstream, and Cisco platforms. Older analog and early digital systems (Nortel, older Toshiba and Samsung units, and similar) do not, but they can often connect through a gateway device that translates between the PBX's analog or PRI ports and SIP. A gateway works, but it is worth being honest about what it means: you are buying new hardware to keep old hardware alive.

Beyond the PBX itself, a SIP trunk deployment needs a reliable internet connection with enough upload bandwidth (each call needs less than 100 kbps), a router that handles VoIP traffic properly (with SIP ALG disabled, the single most common cause of one-way audio), and someone comfortable configuring trunk credentials on the PBX. That last item matters more than vendors admit: with trunking, your PBX remains your responsibility, including its security. Internet-facing PBXs with weak credentials are a favorite target for toll-fraud attackers who rack up international calls overnight.

The honest question: trunk the PBX, or retire it?

Here is the part of this guide a trunking vendor would skip. SIP trunking modernizes the lines; it does nothing for the system. Your auto-attendant, voicemail, call routing, and phones are still whatever your PBX can do, administered however your PBX is administered. No mobile apps, no voicemail-to-email with AI transcripts, no changing your call flow from a browser, no adding a remote employee without a VPN and an afternoon of configuration.

A useful rule of thumb:

Trunking makes sense when the PBX is modern, paid for, natively SIP-capable, and genuinely doing everything you need, and you have someone who can administer it. Then trunks strip out legacy carrier cost while changing nothing else.
Hosted VoIP makes sense when the PBX is aging or already end-of-life (Nortel, Toshiba, and Samsung owners: your manufacturers exited the business years ago, and parts come from eBay), when nobody on staff wants to own a phone server, when your team works from more places than one office, or when the features you want live in the cloud anyway.

With hosted VoIP, the PBX in the closet is retired entirely and the system runs in the cloud: auto-attendant, business-hours routing, voicemail-to-email with AI transcripts, mobile and desktop apps are included per user rather than depending on hardware you maintain. fvoip is a hosted service, priced per user (from $21 per user per month, no contracts, no setup fees) rather than per channel, and we provide free migration help off every major legacy PBX brand: Avaya, Nortel, Mitel, NEC, Panasonic, Toshiba, Samsung, ESI, Allworx, and Cisco. Your numbers port over, and your team's desk phones are either reusable (many SIP models from Yealink, Poly, and Grandstream re-provision to fvoip) or replaceable from $8.99 per month; see supported phones.

The bottom line

SIP trunking is real, mature technology, and for the right business (a solid modern PBX, on-site administration, an expensive PRI to kill) it is a clean cost win. But sizing the decision honestly means comparing three options, not two: keep the old circuits (worst of all worlds, and disappearing anyway as carriers retire copper), trunk the existing PBX (cheaper lines, same system), or move to hosted VoIP (no PBX to own at all). If your phone system is over a decade old, the third option is usually cheaper than maintaining the second, and it is the only one that gets better after you buy it. Talk to us before you sign a trunking contract; the comparison costs nothing, and every fvoip plan carries a 30-day money-back guarantee.