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Glossary · Voice technology

What is SIP trunk?

Reviewed by Sujan ThapaliyaLast updated

Definition

A SIP trunk is a virtual connection that carries phone calls between your phone system and the public network over IP, replacing physical ISDN or PRI lines. Capacity is measured in concurrent calls rather than physical channels.
The three STIR/SHAKEN attestation levels, which underpin how sip trunk behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, sip trunk included.

In practice

How sip trunk actually works

Sizing is about peak concurrency, not handset count. A 40-person office rarely exceeds 15 simultaneous calls.

Voice needs consistency more than bandwidth: about 100 kbps per call with G.711, but jitter and packet loss break calls long before capacity does.

Trunks also fail differently from lines. A physical line degrades audibly; a trunk with an under-provisioned uplink drops calls cleanly at the moment of peak concurrency, which looks like a platform fault and is a capacity one. Sizing for the peak interval rather than the daily average is what prevents it.

Worked example

Ten SDRs making 800 short calls a day rarely exceed 14 simultaneous. Six support agents on nine-minute calls need about the same. Concurrency, not volume, sizes the trunk.

SIP trunk: common questions

How many SIP channels do I need?
One per concurrent call, plus headroom for peaks. Concurrency is not the same as call volume: a team making a thousand short calls a day may never exceed twelve simultaneous, while a support line with long calls needs far more.
What is the difference between a SIP trunk and a phone line?
A traditional line carried one call over dedicated copper. A SIP trunk carries calls as data over your internet connection, with capacity set in software rather than by physical installation — which is why adding channels takes minutes rather than an engineer visit.
Do SIP trunks work with an existing PBX?
Usually yes, if the PBX speaks SIP or sits behind a gateway that does. That is the common migration path: keep the handsets and the PBX, replace the lines behind it, and move the rest later.

Sources

  1. 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.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. 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.

See sip trunk in the product

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.

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