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SIP trunking: connect your PBX to the phone network

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Reviewed by Sujan ThapaliyaLast updated

What is a sip trunking?

SIP trunking carries your phone calls over the internet to and from your existing PBX, replacing physical ISDN or PRI lines. You keep your phone system and your numbers; you drop the copper, the per-channel rental, and most of the long-distance bill.
Why sip trunking works better on numbers you own: on a shared pool your calls are a small fraction of the traffic from the same block, so the caller-ID reputation is mostly other businesses' calling and signing caps at B attestation. On owned numbers the reputation is your own, callbacks reach your team, and calls sign at A attestation with CNAM registered.
SIP trunking runs on numbers you own outright, which is what the diagram's right-hand column describes.

How it works

SIP trunking in four steps

No implementation project. Most teams are dialling the same day they sign up.

  1. 1

    Point your PBX at the trunk

    Register with credentials or authenticate by IP. Works with Asterisk, FreeSWITCH, FreePBX, 3CX, Avaya, Cisco, and anything else that speaks SIP.
  2. 2

    Bring your numbers

    Port existing numbers in, or provision new local, national, and toll-free numbers in over 100 countries.
  3. 3

    Size the channels

    Concurrent-call capacity scales on demand rather than in blocks of 23 like a PRI.
  4. 4

    Fail over

    Multiple points of presence with automatic re-routing, so a single circuit or data-centre problem does not take your phones down.

Capabilities

What you get

Elastic channels

Pay for the concurrency you use. December peaks do not require a permanent upgrade.

Encryption end to end

TLS for signalling and SRTP for media, so calls are not readable in transit.

Attestation A on outbound

Calls from numbers you own are signed at the highest STIR/SHAKEN level, which is what keeps your caller ID from being labelled.

Per-second billing

No rounding a 20-second call up to a minute.

Sizing a trunk: concurrency, not call volume

The single most common sizing error is buying channels against monthly minutes. A trunk carries concurrent calls, and concurrency is a function of call length and arrival rate rather than of total volume.

Outbound teams need barely more channels than seats. Long inbound calls are what consume capacity.
TeamCalls per dayAverage lengthPeak concurrent
10 SDRs, cold outbound8002 min10–14
6 support agents, inbound1509 min6–9
Blended contact centre, 40 seats1,6005 min35–45
Reception only, 3 lines603 min3–5

Add headroom for the peak rather than the mean — the hour that matters is the one where everybody is on a call at once — and remember that a trunk sized to the average will fail every day at the same time and look like a fault.

What to ask a SIP provider before signing

  • Can I port out, and how fast? The answer tells you what leaving will be like.
  • What attestation do calls sign at, and on whose numbers? Full attestation needs numbers provisioned to you.
  • Where does media route? A trunk that hairpins media through the wrong region adds latency you cannot fix locally.
  • What are the concurrency and destination limits, and can I set them? These are your toll-fraud controls.
  • How are rates published? A rate table you can read beats a quote you have to request every time you expand.

Fit

Who a sip trunking is right for

If none of these describe you, a different mode probably fits better; the comparison pages lay out the trade-offs honestly.

  • Businesses replacing ISDN or PRI lines
  • Existing PBX deployments that work and should not be thrown away
  • Call centres needing burst capacity
  • Software vendors terminating their own traffic

Frequently asked questions

How many SIP channels do I need?
Count your peak concurrent calls, not your handsets. A 40-person office rarely exceeds 12 to 15 simultaneous calls; a 10-seat contact centre on a predictive dialer may need 30.
Can I keep my existing numbers?
Yes. Numbers port in, and the port completes on a scheduled date with no downtime window in practice.
What internet connection does SIP need?
About 100 kbps per concurrent call with G.711, less with G.729 or Opus. Consistency matters far more than raw speed. Jitter and packet loss break calls long before bandwidth does.

Sources

  1. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.
  2. North American Numbering Plan Administrator — NANPAArea code assignments, relief planning, and which codes are in service across the North American Numbering Plan.
  3. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.

Try sip trunking without a contract

No seat licences, no minimum commitment. Add a number, top up a wallet, and start. Release the number whenever you like; your remaining balance stays yours.

  • No subscription
  • Owned numbers with Attestation A
  • Compliance built in
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