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SIP Trunking

Reduce costs and increase flexibility with SIP Trunks

SIP trunking is suitable for all businesses, from a single user, to a large call centre servicing thousands of customers.



Businesses are being driven to consider adopting SIP trunking as their first choice for voice traffic, with the threat of the networks turning off traditional ISDN services from 2025.

SIP (Session Initiation Protocol) is an advanced form of voice communications that is delivered across data connections. Traditional telephone calls are converted to SIP, a VoIP standard, which offers more flexibility on how voice calls are handled and reduces costs by eliminating the need for ISDN rentals.

Benefits of SIP over ISDN:

Scalable Unlike ISDN, SIP Trunks are quick and easy to add to your environment to cater for busy periods and often SIP trunks do not have the long term contracts associated with ISDN installations.
Resilience Advanced back-up and disaster recovery options enable calls to be delivered to multiple locations and diverts to be set at extension, reducing the restriction of all calls for the business being directed to just one single number.
Cost savings Eliminate your ISDN line rentals. You can now use the same connection for Voice and data and save on your ISDN rentals.
Flexible Revolutionise the way your staff communicate with the flexibility to have calls delivered to desk phones and mobile devices.


What will I need to benefit from SIP?

To host SIP trunks/voice only you need quality, dedicated broadband or leased line connectivity. This allows SIP to work to its full potential. We would not recommend using your old broadband line. Squeezing voice calls down older, contended broadband circuits means SIP won’t work.

You must have an arrangement with a SIP Trunk provider or a phone calls and line provider that can offer SIP trunks. You pay for installation and rental of every SIP trunk, and for the telephone numbers associated with it.

A phone system that is SIP trunk compatible. It may go without saying, but your phone system needs to be compatible with trunks. Southern Communications can advise on this.

What if my phone system isn’t compatible? Southern Communications ISDN replacement service can use your existing ISDN ports and convert your calls to SIP

Adequate computer network security arrangement. When utilising any communication system over the internet, you need to have a secure computer network to protect your business, employees and clients. This security works to support and allow SIP trunk communications to go ahead.

It’s important to understand that the internet doesn’t prioritise VoIP or SIP phone calls and that you speak to your SIP provider to make sure the quality of your voice traffic is protected from poor quality.


SIP Trunks or Hosted Telephony – which is right for you? 

How does it work?

Voice calls are converted to SIP

  • Existing numbers including geographic and non-geographic numbers can generally be moved to SIP, however we do recommend you check with our sales teams as there a few numbers that cannot be moved to SIP. Directory entries are maintained and 08 and UK numbers can be presented.
  • Calls can be routed to multiple locations for flexibility or as part of a disaster recovery plan in a choice of formats;

    • Secondary failover – calls are delivered to the secondary destination (IP Address) if the primary destination is unavailable
    • Round Robin – alternate delivery of calls between two destinations (IP Addresses)
    • Simultaneous – deliver inbound calls simultaneously to primary and secondary destinations.
  • Moving to SIP is very straightforward. Many businesses already have some or all of the facilities needed to support SIP and even if you don’t have all of them, we can provide the services and advice you need to get everything up and running smoothly. There are two main ways to connect to SIP:

    • ISDN Replacement Service (ESTN) – We simply plug a box into the existing ISDN Card in your telephone systems and it converts your ISDN traffic to SIP, eliminating the need for expensive upgrades to your existing system. Your calls are then routed out of the business over SIP trunks using your data connection.
    • If your telephone system is already SIP enabled we can connect the data pipe into your system and provide as many SIP trunks as you require.

    For moving your voice traffic around your premises your voice devices will need to be connected to your LAN via Ethernet using PoE switches.

  • Most voice services delivered over ISDN/Analogue connections are available on SIP, however there are some restrictions. These include: BT Redcare, 1470/1471, 141 and routing calls through a carrier other than the provider of the data circuit. Before providing a SIP solution we take the time to understand exactly what voice services you are using and how you are using them, to ensure that you do not loose any essential services.
  • Fraud alerts and call barring is managed in the same way as on ISDN circuits to minimise your risk of a fraudulent attack. However, for extra protections Southern Communications VoiceSafe is applied as default.

What data connection do I need?

SIP services can be delivered across a number of data circuits, subject to there being sufficient capacity.

We do not recommend standard broadband connections due to the lack of service guarantees which can result in poor voice quality, as speeds vary depending on usage, not only within your office but also in the local area. Also, there can be long periods of no service in the event of a line fault.

To maintain voice quality connections we recommend allowing a minimum of 100Mbps minimum for 10 concurrent calls and connecting using one of the following circuits;

Ethernet EFM (Dual and Quad) EoFTTC FTTC

QoS enabled v not enabled

SIP will become the standard delivery protocol for voice services and will replace ISDN services. The networks intend to remove ISDN services within the next decade, which will drive all traffic to SIP.

Contact our sales team on 01256 391 046 and upgrade your ISDN services for SIP today.


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