What are SIP Trunks used for?

Posted by SAS on Mar 20, 2019 11:42:00 AM

SIP has fast become a commodity product due to its cost benefits, rapid configuration and business benefits. For those yet to adopt SIP, it’s important to understand the reasons why businesses adopt SIP and how it can help your communications infrastructure.

We’ve highlighted some of the most common uses cases. See if you can spot the ones most pertinent to you.

Cost optimisation

As businesses undergo digital transformation, it’s easy to get carried away with costs. SIP is the perfect technology to bring your business into the 21st century without an increase in expenditure.

Customers moving to SIP trunks for the first time often do so because they have identified the monumental savings associated with them. Per trunk, SIP trunks cost between £1 and £5 dependent on requirements. If you compare this with an average telephone line rental of £17.50 per line, it’s easy to see why businesses are keen to move to SIP trunking.

When a business of 100 employees has an ISDN30 with the full 30 channels, this is a monthly cost of at least £450 per month or £5,400 per year. The SIP trunk equivalent costs out at a no more than £150 per month or £1,800 per year. 

Call costs are typically offered at a reduced rate to calling from the PSTN (landline) network. Calls between sites become free of charge as calls are made from SIP to SIP. This on-net connection routes calls over the internet, ensuring no charges are incurred to call your colleagues in another office.

Further to the ongoing costs, each time you need a new ISDN pair installed, there is an associated installation fee. With SIP trunks, these are configured over the internet and deployed to your existing SIP service. This eliminates the one-off charges each time you need to deploy a new SIP service or add to your existing setup.


When planning for additional capacity in your phone system, there are many considerations. The most painful is exceeding your current capacity. Each ISDN2 circuit you currently have only scales to 8 concurrent calls. Each ISDN30 allows a maximum of 30 concurrent calls.

When planning for – or reacting to – business growth, SIP allows you to instantly scale up your concurrent calls.

Take the case where your business employees more staff for the Christmas period. Seasonal staff require additional phone lines to take incoming calls, as you react to receiving more calls than usual. Capacity planning of old would call for extra ISDN lines to be installed – on a minimum term contract – just to enable these temporary staff to make and receive calls for a month.

SIP offers an excellent alternative to previously a costly exercise. You can scale up when you need further telephony capacity and adapt to ongoing business growth or shrinkage by adding or removing SIP trunks and concurrent calls.

Consolidation and centralisation

SIP can remove the need for centralisation when shrinkage does occur. Communications infrastructure is habitually the first expense to be reviewed.

Moving each site to SIP trunking can recoup the deemed loss made by each site. If you are currently paying for phone line rental, hardware and maintenance, this can be dramatically reduced by stripping it out and replacing with SIP trunks. Rather than needing to close sites, the business case for keeping them open is bolstered by the move to SIP.

Centralisation is common in businesses with multiple locations. SIP trunking removes the requirement for dedicated telephone lines installed at each location. SIP enables consolidation of branch infrastructure and creates a hub like environment for your communications. A centralised master phone system or call manager can route calls to the correct branch over the internet via SIP trunks. This removes hardware and traditional phone line rental costs from each premises.

ISDN replacement

The most widely marketed reason for moving to SIP trunks is the impending shutdown of the ISDN network. By 2025, BT Openreach is expected to turn off the connectivity, in favour of SIP and cloud technologies. Whilst this common marketing message is a clear tactic to encourage businesses to move to SIP trunking, it is also accurate.

ISDN infrastructure will no longer be supported. Businesses will no longer be able to place orders for new or additional circuits.

Businesses will need to be prepared in advance - and most can be today. Connectivity, through dedicated services like Fibre to the Premises (FttP) and SDWAN are already capable of supporting high quality SIP connections. Here, you can apply QoS, network monitoring and proactive configuration changes to ensure SIP is running at peak performance.

For smaller businesses that do not need dedicated services, a common scenario is implementing SIP trunks over a high speed Fibre to the Cabinet (FttC) connection. The costs and complexity of rolling out SIP trunks over broadband remain far less than ISDN implementation.

Cloud readiness

SIP is often viewed as the first step to moving communications into the cloud. As businesses start to trust and invest in the cloud across other technologies like storage, backup and web services, SIP is a natural move for your communications estate.

If your PBX is IP-compatible, switching from ISDN to SIP is as simple as provisioning SIP trunks over your network and moving your recognised phone numbers onto the SIP platform. Once moved, your legacy ISDN infrastructure is fully decommissioned. Your management and administration services are accessible via a browser based portal – like your other cloud applications.

If your PBX is not IP-compatible, you may be the perfect candidate to move straight to cloud telephony. Hosted voice platforms take care of the phone lines like SIP. They also provide the routing and intelligence usually associated with a PBX.

If these use cases are a reflection on the position your business is in, the next step needs to be to find out how many SIP trunks you need and to plan how your SIP implementation will coexist with your network. Read on to find out how to plan your SIP implementation.