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What is a SIP trunk?

A SIP trunk is a phone line delivered over the internet. It connects a phone system you run yourself, such as 3CX, FreePBX or Asterisk, to the public phone network, so every extension can call any landline or mobile.

  1. Your phone system3CX, FreePBX, Asterisk, Yeastar
  2. SIPThe SIP trunkover your internet connection
  3. Any phonelandlines and mobiles worldwide
One trunk carries as many calls at once as you have channels.

Updated 10 November 2026.

SIP

How a SIP trunk works

A handful of short messages set up every call.

SIP (Session Initiation Protocol) sets the call up and ends it; the voice travels separately as RTP packets. "Trunk" is the old telephone word for one line that carries many calls.

  1. INVITEYour phone system asks for a call to +44 20 7946 0000.
  2. 100 TryingThe provider has the request.
  3. 180 RingingTheir phone is ringing.
  4. 200 OKThey picked up.
  5. ACKYour side confirms.
  6. Voice (RTP)Audio flows both ways in small packets.
  7. BYESomeone hangs up. The call ends and billing stops.
1:1

SIP trunk channels

One channel, one call. Count your busiest moment.

Other codecs, a VPN or encryption change the upload you need: VoIP bandwidth calculator.

You need 6 channels and about 0.6 Mbit/s of upload kept free for calls.

One channel carries one call. Each call uses 87.2 kbit/s in each direction with the G.711 codec; download needs the same.

23

SIP trunk vs PRI

A PRI comes in blocks of 23 or 30. A SIP trunk grows one call at a time.

PRI (ISDN) carries calls over a dedicated circuit, sold as a whole T1 (23 channels) or E1 (30). In the UK, Openreach switches ISDN off on 31 January 2027.

PRI for an office that needs 8 lines23 paid, 15 idle
SIP trunk for the same office8 channels, add one when you hire
?

SIP trunk, SIP account or hosted PBX

The right one depends on a single question.

Do you already run a phone system?

You need a SIP trunk. It plugs into 3CX, FreePBX, Asterisk or Yeastar and gives every extension a line to the outside world.

$

SIP trunk costs

Three ways to pay. Only one has a ceiling.

Most providers charge per channel or per minute, plus a monthly fee for each phone number. See what each model costs in a month on our SIP trunk pricing page.

∞

Per channel

$15 to $30

a channel, every month

You rent capacity, used or not. Usually includes domestic minutes up to a fair-use limit.

INV

Metered

$0.005 to $0.02

a minute, invoiced later

You pay for what you use, but there is no ceiling until the bill arrives.

$

Prepaid

Per minute

from credit you load first

You pay for what you use, and spending stops at your balance.

4

SIP trunk security

Toll fraud starts with one guessed password.

Bots scan for phone systems, guess a password and call expensive numbers all weekend. Four locks stop it.

  1. IP

    A firewall

    Only your provider's addresses may reach your phone system's SIP port.

  2. ***

    Unique passwords

    Long, random, and different for every extension and the trunk.

  3. MAX

    Limits

    Calls at once, countries you allow and a cap per day.

  4. $0

    A spending ceiling

    Prepaid credit, so a stolen login can only spend what is loaded.

A

Caller ID and STIR/SHAKEN

In the US and Canada, every call carries a grade for its caller ID.

Under STIR/SHAKEN the carrier that starts a call signs how sure it is of the number shown. A low grade is one reason calls get labelled as spam. In depth: how STIR/SHAKEN works.

A

Full attestation

The carrier knows the caller and that they may use this number.

B

Partial attestation

The carrier knows the caller, but not that the number is theirs.

C

Gateway attestation

The carrier only knows where the call entered its network.

6

How to choose a SIP trunk provider

Six checks before you sign anything.

Use them on any shortlist, ours included: we compare nine providers on our prepaid SIP trunk provider page.

  1. $/min

    Published rates

    A rate per minute for each country you call, mobile and landline, before you sign up.

  2. 6/6

    Billing increments

    Per second or per 6 seconds. Rounding up to whole minutes makes short calls cost more.

  3. +

    International on day one

    Some trunks keep international calls locked until a review.

  4. $0

    A spending ceiling

    Prepaid credit or a hard limit, so a stolen password cannot run up a bill.

  5. PBX

    Works with your system

    Tested settings for 3CX, FreePBX, Asterisk or whatever you run.

  6. 911

    Emergency calls, if you need them

    Not every trunk carries them. Ask, and keep another line if it does not.

4

How to set up a SIP trunk

Four settings from your provider. One dial rule.

Every phone system names them differently. Our SIP trunk configuration guide shows where they go in 3CX, FreePBX, Asterisk and Yeastar.

What a SIP trunk login looks like
Server
sip.example.com
Username
100234
Password
a long random string
Transport
UDP, port 5060
Dial rule
send numbers as +, country code, number
FAQ

SIP trunk questions

What does SIP trunk mean?
SIP is the Session Initiation Protocol, the internet standard for starting and ending calls. A trunk is a line that carries many calls at once. Together: a phone line for a phone system, delivered over the internet.
What is the purpose of trunking?
Sharing. Instead of one line per person, a trunk gives a whole office a pool of channels, sized for the most calls you make at the same moment.
Is a SIP trunk the same as VoIP?
A SIP trunk is one kind of VoIP. VoIP means any voice over the internet; a SIP trunk is the specific service that connects your own phone system to the phone network.
What does SIP mean on a phone?
A phone or app that says SIP can make calls over the internet by logging in to a SIP provider or a phone system, instead of using a mobile network or a copper line.
Who are the top SIP trunk providers?
Large ones include Twilio, Vonage, Bandwidth and Telnyx. We compare nine on price, international access and fraud protection on our SIP trunk providers compared table.
Do I need a special internet connection?
No, but you need steady upload as well as download: about 87 kbit/s each way for every call at once with G.711, and a router that does not interfere with SIP.
Can a SIP trunk make emergency calls?
Some can, where the provider registers your address. LinkedSIP does not carry emergency calls, so keep another line for them.

A SIP trunk is four settings. Yours could be ready tonight.

No channel fees, no ID, international calls from your first top-up.

See the SIP trunkSee call rates