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How a SIP Trunk works

Functionality of a SIP Trunk

SIP stands for Session Initiation Protocol and describes a network protocol with which a communication session (usually a telephone call) can be controlled. VoIP (Voice over IP) is usually based on this protocol. With a normal VoIP connection, each terminal or telephone number has its own SIP account.

With a SIP trunk you can use several phone numbers bundled in a direct dialable number block with only one account. Only a central IP-based telephone system (IP-PBX) registers and forwards the call to the extensions (SIP-DDI). The configuration of a SIP trunk is therefore quite simple. Even for outgoing calls, the telephone system decides which numbers are shown to the called party (CLIP no screening).

Internal company calls run over the intranet of your company, so your Internet connection is not burdened by additional calls.

That's why you need SIP Trunks

Good reasons for the drink

IP-based telephony is becoming more and more important due to the shutdown of classic ISDN connections, which is to be completed by 2018.

For companies, SIP trunking is the proven successor technology to the now outdated ISDN and primary multiplex standards.

Advantages of a SIP Trunk

SIP Trunking connects your telephone system simply, conveniently and reliably to the telephone network. You have no loss of quality to the ISDN or primary multiplex connection.

  • Thanks to HD telephony, the call quality is often even better than with the old connection technologies.
  • In addition, you can also encrypt your calls so that no third party knows who you are talking to and what you are talking about. With ISDN and primary multiplex it is not possible to encrypt calls.
  • A SIP trunk provider like easybell also offers you an easy-to-administer online menu. You always have your phone numbers and call lists in view and control many functions such as conference rooms, call forwarding or number display simply online.
  • Functions such as SMS or FAX dispatch are also available online.
  • IP-based telephone connections are also usually much cheaper than traditional telephone connections. With easybell, for example, you pay no basic fee for a connection with two lines in the first year and then only € 1.67 per month.
  • With our business tariffs, you enjoy all the advantages of SIP trunking. So you can safely say goodbye to the ISDN of the past. Decide now!

How to switch to SIP Trunking

A change is very easy and your phone number will be reachable without interruption. You simply order a SIP Trunk and set it up in your IP-enabled telephone system. So you can benefit from the lower minute prices of IP telephony right from the start. Then you order the porting of your ISDN telephone numbers. We as your SIP Trunk provider will then take care of the rest for you. All you have to do is make sure that you have an Internet connection.

In addition, we offer you the purchase of our easybell IP telephone for our business rates - your guarantee for elegant business telephony in top HD quality.

For telephony, you have to allow for approx. 100 kbit/s per simultaneous call. From the day of the porting, the incoming calls will also run via the SIP Trunk and you have switched to the telephony technology of the future.

Excursus: What you should consider when buying a VoIP-TK system

Telephone system manufacturers offer a wide range of products, which can vary greatly in price and performance. It is good to know beforehand which requirements you would like to make on a model at which conditions. For small companies with up to five workstations, a FRITZ!Box usually covers the required functions. This is often already in use as a DSL modem. Which brings us to the first important point for the selection of a telephone system: the size of the company. Because different systems can be considered depending on the size.

Still important:

  • Functional requirements
    Is it enough if it is easy to make phone calls or do I have to run all my alarm systems over the PBX?
  • General handling
    Do I have in-house specialists or do I need professional help with the implementation?
  • Investment costs
    What can my business expect and how can I avoid unnecessary costs?

But first it is important that you ask yourself basic questions: Should your new telephone system be inexpensive and at the same time offer free manufacturer updates? Is it important that you can make a lot of calls at the same time and have the ability to record your calls? Are complex dialing scenarios important, but also video telephony? A good provider can help you find answers to these questions.


The selection of the SIP trunk provider is also extremely important because it will be the one with whom you will regulate your business telephony in the future. For this you need not only a partner at your side who is absolutely reliable but also secure. easybell stands for this security and reliability. We have been working with VoIP sales for years and therefore know what is important. Our easybell SIP Trunks increase the accessibility and communication possibilities of your company. Thanks to its flexible “switching options”, any number of trunks can be set up and switched to another trunk as required. And: easybell has certified numerous well-known telephone system manufacturers. This means that we have tested the compatibility of our SIP trunks in advance and can guarantee it for you.


Who can help with the configuration of my telephone system?

easybell naturally helps you to configure your telephone system with our SIP Trunks. Here you will find clear instructions for numerous telephone systems.

Can I take my phone numbers with me to easybell?

Yes, porting to easybell is very easy. You can take your previous number with you both to our complete connections and to our VoIP connections. All further information on the subject of “porting” can be found here.

Further FAQs can be found in our easybell Help Center.

Small glossary

CLIP no screening
With CLIP no screening, the caller transmits customer-specific telephone number information without the transmitted telephone number being checked for correctness by the switching telephone network. You can then, for example, display a service number to the called party instead of your own. In the migration process, for example, only the outgoing telephony is initially managed via VoIP and, after successful completion, the incoming telephony is also taken over.

Codec refers to a fixed method for data encoding and decoding. A well-known codec is e.g. MP3 (MPEG-1 Layer III). Codecs are also used for digital telephony. easybell supports the G.711 codec, which is also used for ISDN.

DSL stands for “Digital Subscriber Line” and describes a series of transmission standards in which data can be transmitted at high transmission rates over the conventional copper telephone line.
easybell offers you the two advanced DSL variants ADSL2+ and VDSL2.

The Session Initiation Protocol (SIP) is a network protocol used to establish, control and terminate a communication link between two or more participants. It is mainly used in VoIP telephony and is also the basis for the easybell telephony service.

SIP Trunking
A SIP trunk can be considered as a virtual physical channel or a virtual telephone line connecting a telephone system to the SIP provider. Signalling and voice streams are transmitted over the Internet. A SIP trunk can transmit many calls simultaneously. A SIP trunk can be used instead of a conventional ISDN line or as a supplement to existing ISDN lines.

VoIP stands for “Voice over Internet Protocol”. VoIP is also called IP telephony in German-speaking countries.
With VoIP, the voice connection is established via the Internet. VoIP is the common telephony technology today and offers numerous comfort features that are not possible with classic telephony.
You can make calls with VoIP just as with the old connection technology. All you need is an Internet connection and a VoIP-enabled terminal device (e.g. FRITZ!Box).
In contrast to the frequently mentioned NGN VoIP services of many providers, easybell VoIP is completely open and can be used via any broadband connection.


To the large glossary