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SIP Trunking

Business Phone Systems

SIP Trunking for Businesses With an Existing PBX

Replace the lines into the PBX you already own, without replacing the phone system itself.

What is SIP trunking and how is it different from a hosted phone system?

SIP trunking replaces the traditional phone lines feeding a phone system you already own, delivering calls over your internet connection instead of over copper lines. A hosted phone system replaces the phone system itself, moving the call-routing equipment into the provider's data center. The distinction is what you keep: with SIP trunking the PBX in your building stays and only the lines change, while a hosted system makes that hardware redundant. SIP trunking is billed per channel — one channel carries one simultaneous call — rather than per user, which is why it often costs less for businesses with more staff than concurrent calls.

Channels, not users

This is the part businesses most often get wrong when sizing SIP trunks. A channel carries one call at a time, so the number you need is your peak simultaneous calls, not your headcount.

A thirty-person office where six people are typically on the phone at once needs roughly six to eight channels, not thirty. That arithmetic is why SIP trunking frequently costs less than per-user hosted pricing for businesses with a low ratio of concurrent calls to staff.

When SIP trunking is the right answer

  • Your PBX still does what you need. Replacing functional hardware rarely makes financial sense.

  • You have more staff than simultaneous calls. Per-channel pricing rewards that shape.

  • You want to modernize the lines without a project. Changing trunks is smaller in scope than replacing a phone system.

When it is not

  • The PBX is at end of life. Putting new trunks into hardware about to fail defers a decision rather than making one.

  • Staff need to work from anywhere. A hosted platform handles distributed working more naturally.

  • Nobody wants to maintain hardware. SIP trunking keeps that responsibility with you.

The compliance point

Kari's Law and RAY BAUM's Act apply to multi-line phone systems whatever platform they run on: 911 must be dialled directly with no prefix, and a dispatchable location must travel with the call. What changes with SIP trunking is where the responsibility sits. A hosted provider handles most of it for you; when you keep your own PBX, more of that configuration — and more of the obligation — stays with your business.

What to check first

Compatibility. NVTGI places SIP trunking against any SIP-capable PBX. Older systems may need a gateway or session border controller to accept SIP trunks, which adds hardware cost and changes the economics of the project — so we assess the specific system before quoting rather than after.

Worth stating plainly as well: SIP trunk calls run over your internet connection, so call quality and continuity are tied to that link. Where the connection is marginal, fixing it comes before the trunks do.

Also worth comparing: GoToConnect and Broadvoice.

Best suited for

Businesses with working PBX hardware that want lower line costs without replacing a system that still does the job.

What you get

Keep the PBX you own

Only the lines change; the phone system in your building stays.

Per-channel pricing

Sized to peak simultaneous calls rather than headcount.

Numbers port across

Existing business numbers move onto SIP trunks the same way they move between carriers.

Common questions

Yes — that is precisely what SIP trunking does. The PBX in your building continues to handle extensions, call routing and voicemail, while the lines feeding it are delivered over your internet connection instead of over traditional copper. Industry consensus is that businesses with functional PBX hardware benefit most from this approach, because scrapping equipment that still works rarely makes financial sense. The main compatibility question is whether your PBX accepts SIP natively or needs a gateway in front of it.

SIP trunking is priced per channel, where one channel carries one simultaneous call. The number of channels you need is driven by peak concurrent calls rather than by headcount — a thirty-person office where six people are typically on calls at once needs roughly six to eight channels, not thirty. Sizing is usually based on historical call records from the existing system, with headroom for busy periods. We quote per-channel pricing once we know your peak concurrent call volume.

Frequently yes, for businesses with an existing PBX and a low ratio of simultaneous calls to employees, because you pay per channel rather than per user. Hosted VoIP across the industry runs roughly $10 to $50 per user per month, so a business with many staff and few concurrent calls pays substantially less on channels. The trade-off is responsibility: with SIP trunking you continue to own, maintain and eventually replace the PBX, along with its configuration and its compliance obligations. Hosted systems cost more per user but move that burden to the provider.

Most modern PBX systems accept SIP trunks natively, and NVTGI places SIP trunking against any SIP-capable PBX. Older systems designed for traditional lines usually need a gateway or session border controller in front of them to translate between the two, which adds hardware cost and a configuration step — we assess that up front rather than discovering it mid-project. On an older PBX that additional cost is occasionally large enough to change the decision between trunking and replacing the system, and we will say so if that is what the assessment shows.

Voice calls are undemanding individually — roughly 100 kilobits per second per concurrent call — so eight channels consume under one megabit of capacity. Raw bandwidth is rarely the constraint. What matters far more is consistency: calls degrade when the connection has jitter or packet loss, even with plenty of capacity spare. This is why voice quality is usually better on a connection with modest speed and stable performance than on a fast connection shared with heavy traffic, and why quality-of-service configuration on the router matters more than buying a bigger plan.

Calls stop arriving at the PBX, because the trunks are delivered over the internet connection. Every SIP provider can configure automatic failover routing that redirects inbound calls to mobile numbers or an alternate location when the trunk is unreachable, so calls continue to reach staff rather than failing outright. The stronger arrangement is a backup internet connection — commonly cellular failover — so the trunks themselves stay up. Businesses moving from traditional lines to SIP should plan this explicitly, because copper lines survived power and internet failures in a way SIP trunks do not.

Emergency calling works over SIP trunks, and the federal obligations under Kari's Law and RAY BAUM's Act apply to multi-line telephone systems regardless of the technology delivering the lines. Kari's Law requires direct 911 dialling without a prefix and notification to designated on-site personnel; RAY BAUM's Act requires a dispatchable location precise enough to find the caller inside the building. When you run your own PBX, more of that configuration is your responsibility than it would be on a hosted platform, because the routing and location data are held in your system. The registered address for each location must be kept current.

Yes. Existing business numbers port onto SIP trunks the same way they move between any two carriers, typically taking 7 to 14 days across the industry, with the existing service remaining live throughout. Main lines, direct dial ranges and toll-free numbers all port. Because the PBX keeps handling the routing, extension numbers and internal dial plans usually stay exactly as they are — staff do not need to learn anything new, which is one of the practical advantages of trunking over replacing the system.

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