Frequently Asked Questions about SIP Trunking

Q: What is SIP?

A: SIP stands for Session Initiation Protocol. It is designed to set up connections between devices such as computers, phones, PBXs, video conferencing units and servers.

Q: What is SIP Trunking?

A: A SIP Trunk is a virtual phone line that connects your phone system to the public telephone network. It sends the data over a data network, instead of over an ISDN circuit. SIP Trunks can be used instead of ISDN circuits to carry all of your telephone calls, or in conjunction with an ISDN circuit to provide a resilient telephony solution.



Q: What are the advantages of using SIP Trunking rather than ISDN?

A: SIP Trunks utilise your existing data connection, thereby negating the need to spend money on monthly line rental for ISDN circuits. Calls sent over the Internet are much cheaper than calls sent over traditional telephone infrastructure, thereby saving on your monthly call costs. Because SIP Trunks are provisioned over your existing data connection, provisioning can be done instantly as you do not need to wait for circuits to be installed.

Unlike with ISDNs where you have a maximum of 30 channels per circuit, a SIP trunk is fully scalable so you can have as many or as few as users as you want.

By retaining one of your old ISDN circuits, you can add resilience to your phone system, thereby ensuring that even if one of your connections goes down, your telephones will continue to work at all times.

Q: When should a business use SIP Trunking?

A: SIP Trunking is relevant to your business if you use ISDN2 or ISDN30 circuits, and if you get your Internet connectivity through a leased-line or a dedicated SDSL.
SIP Trunking is irrelevant to your business if your business is too small to use ISDN circuits, and if you use contended ADSL or Wi-Fi to provide your Internet access.

Q: What equipment do I need to set up SIP Trunking?

A: If your phone system is older than a year or two, you probably need a little box called an SIP-ISDN Gateway. It acts as a translator between the SIP of your SIP Trunks, and the ISDN interface of your phone system.
If you have an IP-PBX made in 2008 or later, it probably supports SIP, without any need for a gateway.

Q: Will I need additional connectivity to use SIP Trunking?

A: Yes. Although it is possible to run SIP Trunks over the public Internet, we wouldn’t advise it. Phone calls are sensitive to delay, variations in delay and data loss (aka latency, jitter and packet loss). For this reason, we never sell SIP Trunks without connectivity.

Q: How many SIP Trunks will I need?

A: That depends on how many simultaneous calls you expect to take place at one time. Obviously, this will depend on your business. If you’re running a call centre it could be one SIP Trunk per operator. For more typical companies, you’d usually have one SIP Trunk for every 5 or 6 phones.

Q: Are SIP Trunks reliable enough?

A: Set up correctly, SIP Trunks are more reliable than ISDN. Set up incorrectly, they’re less reliable. The key thing isn’t the ‘SIP’ bit, but the ‘data connectivity’ bit over which the SIP Trunk runs. To get good reliability you need a direct connection to your SIP Trunk provider. You also need there to be a backup connection of some sort.

Q: Are SIP Trunks expensive to implement?

A: Chances are it will save you money.
Up front, you’ll need to pay for a direct connection to your SIP Trunk provider (to ensure good call quality). You’ll probably also need a SIP-ISDN gateway.

As soon as the service is live you’ll save money on your connectivity (because the data connection will be cheaper than your existing ISDN circuit rental). You’ll also save money on your line rental and on your calls.

So the switch from ISDN30 to SIP usually pays for itself.

Q: What services will SIP support?

A: Most of the functionality you associate with your phones is really provided by your PBX. So whether you’re using SIP trunking or ISDN trunking makes no difference.
Our SIP Trunks provide access to UK emergency services (i.e. 999 works). Not all SIP Trunk providers offer this.

For more information, give us a call on 0800 652 7681 or email