How a SIP trunk connects an office to the public phone network
A SIP trunk is the set of outside lines that connects an office phone system to the public phone network over the office's internet line, instead of over lines from the telephone exchange. The phone system signs in to its provider, and each call travels to the provider's data centre as SIP messages and packets of voice, where a session border controller checks it and hands it on. It replaced ISDN, whose lines carried 2 or 30 calls on fixed channels; on a SIP trunk, the number of calls at once is a figure in the contract.
Purple: the call and the parts it usesKey
- Purple: the call and the parts it uses
- Lifted handset: on the phone
- Dashed: the mobile network, through the air
SIP trunk · the outside lines run over the office's internet line
Press a button, or click a desk phone to call from that desk.Press a button, or tap a desk phone to call from that desk.
The same office and the same phone system. Only the outside lines changed, from an ISDN line with fixed channels to a trunk over the internet line.
Anatomy of a SIP trunk: one street, from the phone system in your office under the street to your provider's data centre and on to the telephone exchange, on a SIP trunk and on an ISDN line. The drawing is not to scale.
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Follow one call
Press Play to watch the call, or scroll down and the drawing follows you one step at a time. Try it, under the drawing, makes your own call.Press Play to watch the call, or Next to go one step at a time. Try it, under the drawing, makes your own call.
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Step 1 of 6
How the network knows your phone system
Your office's phone system sits in the cabinet in the corner. It knows every desk phone and every extension, and it holds the outside lines for calls to the rest of the world. The network has to know it too. On a SIP trunk those lines run over the office's internet line, so the phone system signs in. When it starts, it sends a short message through the router and under the street to your provider's data centre. The message says that this phone system is here and which numbers are its own. On the ISDN line there was nothing to sign in to. The exchange knew your line by the copper wires that ended on its frame.
Does every SIP trunk sign in like this?
No standard defines a SIP trunk. Providers follow SIPconnect, a document from the SIP Forum, an industry group. It describes two ways. In registration mode the phone system signs in, as here, so it can sit behind an ordinary internet line whose address changes. In static mode the provider enters the phone system's fixed address once, and the phone system never signs in.
The IETF, which writes the internet's standards, described how a phone system signs in for a whole block of numbers in 2011. Deutsche Telekom's trunk specification strongly recommends registration mode.
Sources: NICC ND1034: UK SIPconnect endorsement (2024); IETF RFC 6140: registration for multiple phone numbers in SIP (2011); Deutsche Telekom: 1TR118, technical specification of the SIP-trunking interface (2019)
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Step 2 of 6
You call a customer, and the call leaves the office
You lift your handset and dial a customer's mobile. Your desk phone passes the number to the phone system. It sees an outside number and sends a call request out on the outside line. On the SIP trunk the request, called an INVITE in SIP, goes through the router, down the wall, under the street and up into the data centre, on the same internet line that carries the office's email. On the ISDN line the request left on the line's own signalling channel, beside the channels that carry voices, and ran under the street to the exchange.
Is an office internet line fast enough for calls?
A call needs little of it. With G.711, the phone network's standard way of turning a voice into numbers, a call sends 50 packets a second each way, each with 20 milliseconds of voice. Every packet holds 160 bytes of voice and 40 bytes of addresses and numbering, so one call needs 80,000 bits a second each way.
Deutsche Telekom tells its trunk customers to plan for about 138,000 bits a second per call, because the line adds its own framing. What a call needs most is a delay that stays short.
Sources: ITU-T G.711: pulse code modulation of voice frequencies (1988); IETF RFC 3551: the RTP profile for audio and video (2003); IETF RFC 3550: RTP, voice and video in packets (2003); IETF RFC 768: the User Datagram Protocol (1980); IETF RFC 791: the Internet Protocol (1981); Deutsche Telekom: DeutschlandLAN SIP-Trunk, technische Unterlage (2023)
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Step 3 of 6
The network checks the call at its door
Where your line enters the network, your provider checks the call. On the SIP trunk this happens in the data centre, at the session border controller, the provider's guard at the door. It checks that the request comes from a phone system it knows. It also checks the number you show the customer. The phone system puts one of your numbers in the request, and the provider makes sure it is yours to show. The border controller also hides the provider's own machines from the internet, and keeps the way back open through your office router. On the ISDN line the wire did this job. The exchange knew which copper pair the call came in on.
What happens if someone breaks into the phone system?
They can use your trunk. The UK's National Cyber Security Centre warns that criminals who get into an office phone system dial expensive international or premium-rate numbers through it, often hundreds or thousands of times. The provider may spot it, but it expects the owner of the phone system to pay the bill.
Its advice: change the default passwords, accept traffic only from trusted addresses such as the trunk provider's, and block international, premium-rate and out-of-hours calls that the office does not need.
Sources: National Cyber Security Centre: private branch exchange best practice (2024)
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Step 4 of 6
A switch hands the call to the phone network
A switch reads the number you dialled, finds the network that serves it and hands the call over. On the SIP trunk that is your provider's call switch, behind the border controller. On the ISDN line it was the exchange's own switch, in the racks along its back wall. Here the customer's mobile is served from the telephone exchange down the street, which sends the call up its mast and over the air. The mobile rings. When the customer answers, your voices travel the same way in both directions, as packets on the SIP trunk.
How does the switch know which network a number belongs to?
People keep their number when they change operator, so the number alone does not say which network serves it. In Luxembourg, for example, every landline number that has moved sits in a shared database, with the routing prefix of its new operator. A switch can look the number up before it hands the call over.
The European Union's telecom code says a number you move must work at the new provider within one working day of the agreed date.
Sources: Legilux: Règlement 16/204/ILR on fixed number portability (2016); EUR-Lex: Directive (EU) 2018/1972, the European Electronic Communications Code (2018)
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Step 5 of 6
A customer calls Eva's direct number
Your outside lines come with a block of numbers: one for the office and one for each desk. A customer dials the office number that ends in 202, Eva's extension. The public phone network sends the call to your provider, because the number is in its block. On the SIP trunk the border controller passes it over your internet line to the phone system. On the ISDN line the exchange sent the last digits down the line. Either way, the phone system picks the desk and rings Eva.
Where do the numbers of a SIP trunk come from?
They are the same kind of number as before. ISDN called the service direct dialling-in: a caller on the public network reaches a desk behind an office phone system by its own number.
A SIP trunk keeps it. Deutsche Telekom sets up a trunk with blocks of numbers, both on its network and on the phone system, and the phone system signs in once for all of them.
Sources: ETSI EN 300 064-1: ISDN direct dialling-in (1998); Deutsche Telekom: 1TR118, technical specification of the SIP-trunking interface (2019)
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Step 6 of 6
Three people call out at once
You, Eva and Sam all pick up and call out at the same time. Switch to the ISDN line first. This office's line has two channels, so Sam's call finds none free and gets no further than the phone system: his screen says the lines are busy. A bigger office took a line with 30. On the SIP trunk the three calls share the internet line, each as its own stream of packets. How many can run at once is a number in the contract, which the provider checks as each call is set up, and the line must be fast enough for all of them.
What if the trunk is full and someone dials 112?
The call still goes through. SIPconnect says a provider must not apply its limits to emergency calls. Deutsche Telekom goes further: if the line has too little room, it ends another call at random to make room for the emergency call.
The European Union's telecom code says every user must be able to reach the emergency services on 112 free of charge, and the caller's location must reach the emergency centre as soon as the call is set up.
Sources: NICC ND1034: UK SIPconnect endorsement (2024); Deutsche Telekom: DeutschlandLAN SIP-Trunk, technische Unterlage (2023); EUR-Lex: Directive (EU) 2018/1972, the European Electronic Communications Code (2018)
The ISDN line and the SIP trunk, side by side
The phone system, the desk phones and the numbers stay the same in both. What changed is the line between the office and the network, and where it ends.
ISDN line
SIP trunk
What carries the callsITU-T I.412; NICC ND1034
Its own line
From the telephone exchange, for calls only
Internet line
The same line as the office's data
Calls at onceITU-T I.412; Deutsche Telekom
2 or 30
Fixed channels: a basic rate or a primary rate line
2 to 300
A number in the contract, at Deutsche Telekom
Room for one callITU-T I.412; IETF RFC 3551
64 kbit/s
One channel, held for the whole call
80 kbit/s
Each way, as packets, with G.711
How the network knows it is youNICC ND1034; IETF RFC 6140
The wires
Your line ends on a port at the exchange
A sign-in
Or a fixed address the provider knows
SIP trunk in numbers
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1972
Every SIP trunk that follows SIPconnect must still carry G.711, a way of turning a voice into numbers that was first agreed in 1972.
ITU-T G.711: pulse code modulation of voice frequencies (1988)
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1 million
In its first month of blocking calls from abroad that faked UK landline numbers, BT stopped up to 1 million such calls a day.
Ofcom: new blocking requirements to protect consumers from scam calls from abroad (2025)
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19,000
In France, reports of faked caller numbers rose from 531 in 2023 to more than 19,000 in 2025.
How it came to be
- 1984
ISDN is written down
The international body for telecom standards sets out the ISDN line: a basic line with two channels for calls, and a primary line with 30 in Europe and 23 in North America. Each channel carries one voice as 64,000 bits a second.
ITU-T I.412: ISDN interface structures and access capabilities (1988)
- 2002
SIP takes its current form
The IETF, which writes the internet's standards, publishes SIP in its current form, replacing the first version of 1999. A phone system can now set up calls over the internet and tell a server where it is.
- 2016
One recipe for trunks
The SIP Forum ratifies SIPconnect 2.0, its profile for connecting an office phone system to a provider's network with SIP.
- 2019
France stops selling ISDN
Orange stops selling Numéris, the French ISDN, in mainland France on 15 November 2019. Lines already in place keep running until at least the end of 2023.
- 2020
Germany moves its lines to IP
In March 2020 Deutsche Telekom says it has moved 25 million customers, 99% of them, from analogue and ISDN lines onto its IP network, and that the rest will follow within the year.
- 2022
ISDN ends in the Netherlands
KPN stops ISDN completely on 1 April 2022 and offers SIP trunks and internet calls in its place.
- 2027
The UK switches ISDN off
BT plans to switch off ISDN, with the old landline network, by 31 January 2027.
Who may run your trunk, and on what terms
A SIP trunk gives you phone numbers and calls to and from the public phone network, so in law it is a telecom service. In the European Union the European Electronic Communications Code sets the rules, and each country's regulator enforces them. Several countries add their own rules on the number a caller shows.
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It tells the regulator before it starts
The Code calls a service whose numbers reach the public phone network a number-based interpersonal communications service. Its provider needs no licence to wait for, but a country may ask it to notify the regulator before it starts. Luxembourg and Belgium both do.
EUR-Lex; Institut Luxembourgeois de Régulation; Belgian Institute for Postal Services and Telecommunications
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Emergency calls get through, with your location
Every user must be able to call 112 free of charge, and the caller's location goes to the emergency centre as soon as the call is set up. Countries must also promote 112 from private networks, such as the one behind an office phone system. Step 6 explains what happens on a full trunk.
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The number you show must be yours
In Germany, since 1 December 2022, an office phone system may send only numbers its owner has the right to use, and a German number on a call from abroad is hidden. In France the law of 24 July 2020 makes operators check the caller's number and cut calls that fail, and the check began rolling out in October 2024. In the United Kingdom, since 29 January 2025, providers must block calls from abroad that show a UK landline number.
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Your numbers go with you
Anyone with a number from a national numbering plan may keep it when they move to another provider. Once the move is agreed, the new provider must have the number working within one working day of the agreed date.
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In the United States, the caller's number is signed
The Federal Communications Commission made providers sign and check the caller's number on calls over internet networks by 30 June 2021. The system is called STIR/SHAKEN, and its signatures are written to IETF standards.
Where Voxbi fits
Voxbi is a cloud phone system hosted in the European Union, so the phone system sits in the data centre beside the trunk, not in your office cabinet. Mixvoip, the telecom operator that owns Voxbi, provides the numbers and the trunk to the public phone network. Voxbi can also connect a SIP trunk you bring from another provider.
See it in Voxbi