Session Initiation Protocol (SIP)

What is it?

Session Initiation Protocol (SIP) is the next logical step in the evolution of voice communications; routing voice calls over the internet (Voice over Internet Protocol or VOIP) providing a feature rich and cost effective service.

SIP is a signaling protocol, widely used for setting up communications sessions. A session could range from a simple telephone call to a multi media conference call over the Internet. Other application examples include; video conferencing, streaming multimedia distribution, instant messaging, presence information and online games. In November 2000, SIP was accepted as a 3GPP signaling protocol and permanent element of the IMS architecture for IP-based streaming multimedia services in cellular systems.

Microsoft instant messaging and the G3 GSM network are both based on SIP.

At the residential end of the market, Voice over Internet Protocol (VOIP) using SIP technology, has already taken off with products such as Skype. There have been 100 million downloads of Skype software to date enabling millions of people to make free calls between friends and family over the internet across the world.

Features and Functionality

SIP provides a signalling and call setup protocol for IP-based communications that can support a superset of the call processing functions and features present in the public switched telephone network (PSTN). SIP itself does not define these features; rather, its focus is call-setup and signalling. However, SIP has been designed to enable the building of such features in network elements known as Proxy Servers and User Agents. These are features that permit familiar telephone-like operations: dialling a number, causing a phone to ring, hearing ring back tones or a busy signal. Implementation and terminology are different in the SIP world but to the end-user, the behaviour is very similar to standard telephony.

What is a SIP Trunk?

A SIP Trunk is the equivalent of an ISDN channel, or for home-based workers, a broadband connection.

In many cases, SIP Trunks allow businesses to retain the advantages of their existing telecoms equipment, whilst gaining the additional functionality and benefits of IP Telephony. Your organisation may be able to retain existing handsets and have the flexibility to add remote or central IP users when needed - all routed via our Tier 1 carriers.

What are the benefits of SIP?

Our services offer the following:

  • Very competitive tariff

One of the key benefits of using SIP trunks is cost-savings. By replacing your legacy ISDN or analogue telephone lines with more cost- effective SIP Trunks there are potentially significant savings to be made.

  • Any number anywhere

Almost any geographic number range can be routed to your organisation.

  • Number portability

There is also flexibility in terms of numbering – should your business relocate, your telephone numbers can move with you.

  • Multi-site savings

Cost savings can be made in implementing a SIP trunk connection, especially if deployed across multiple sites, where site to site calling cost reductions can apply.

  • SIP to SIP calls free of charge

Calls to other organisations using registered SIP trunks are free of charge.

  • Disaster recovery is a key application

SIP Trunking solutions can offer a very flexible set up to provide Disaster Recovery / Business continuity solutions to customers. By creating a SIP account for a customer’s PBX the telephone numbers associated with that account are now hosted with the SIP providers network.

In the event of a disaster that makes the site inaccessible, the account can be utilised in another PBX on another site and connected via an IP connection - the service can be recreated instantly and seamlessly. Alternatively the PBX itself could be moved to another site any with an IP connection and service can automatically be restored.

There is no need to re-provision the network service and there are no additional network charges or remote call forwarding costs to pay.

Frequently asked questions

What level of voice quality can I expect using SIP?

With the correct configuration you can expect quality equivalent to that of an ISDN line when using SIP trunks.

How reliable are SIP trunks and VOIP?

The major benefit of the internet is there is no single point of failure. ITCs service providers have equipment based in separate locations; should one connection fail the call will be automatically routed to an alternative location. SIP service has a reliability comparable with traditional phone lines.

Can I terminate my existing service?

We would always recommend you keep a percentage of your analogue or ISDN telephone lines as back up and for emergency service calls.  In the event of your broadband connection going down, we will route calls to an alternative telephone number/s.  Most VoIP Phone systems are configured to use existing Analogue and ISDN Telephone Lines as well as SIP Trunks. What level of voice quality can I expect using SIP?

With the correct configuration you can expect quality equivalent to that of an ISDN line when using SIP.

Are emergency services supported by SIP?

No, emergency service are not supported by SIP. It is essential that you have at least one analogue line for emergency services. This can be the line over which your ADSL connection is hosted.

Why ITC?

While there are many SIP providers in the UK, we lead the way for business customers with PBXs using multiple trunks. We have tested extensively with PBX manufacturers and can provide a smooth transition from ISDN to SIP.

What do I need to get started?

You need a SIP enabled device (PBX or handset) and a broadband internet connection, such as ADSL.

Can we keep our old number with SIP?

Yes, your existing telephone number can be ported so that you can receive incoming calls via SIP.

What telephone number will we be given?

New numbers can be ordered if required. You can select which area code you would like, so that you can have a local number.

Is fax supported with SIP?

No, fax is not currently supported over SIP at this time. It is a new technology and ITC plan to add this service in the future.

If you would like one of our telecommunications consultants to contact you for further advice, please email with your details.

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Please Note - To ensure ITC provide our valued customers with the highest levels of service our minimum call threshold per client is 5,000 call minutes per month.

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