The business case for SIP

Times, they are a changing.

Over the past few decades, cultural and technological trends have dramatically transformed the workplace. Today, flexible working is a way of life. Employees now have greater control over their work-life balance than ever before, and employers understand that productivity is not confined to a desk. As such, adopting technology that enables a mobile workforce
has become a business prerogative. 

Yet this need for new mobile tools is often at odds with other business demands. While many senior management teams are keen to stay ahead of the competition, they also want to keep costs down.

Every investment is scrutinised, and new equipment or systems purchased today must adequately cope with depreciation and tech innovation long into the future. 

SIP trunking is a future-forward solution. Because it enables business agility, scalability and can bring about immediate cost savings, it is the ideal answer to these conflicting sets of business demands. 

This Guide looks at how SIP can help businesses cope in a changing world – taking into consideration the technicalities of installation, and how preparation is key for seamless telephony success.

Three questions, same answer

All businesses have different priorities. Some may be voice-critical, with the smallest period of downtime leading to disastrous results. Others may be heavily influenced by seasonal demands, so telephony systems must be easily scalable. Or, they may simply need to keep costs down.
Whatever the situation, SIP has something to offer.

No physical lines

Because SIP trunking uses virtual lines rather than physical wires, businesses have greater control over their telephony infrastructure. This means having the power to consolidate voice and data over a single network, increase resilience, and put in place flexible inbound call routing (i.e. to mobiles or alternate office sites) so that calls are never missed.

Highly scalable

SIP trunking’s virtual nature means it is easy to scale the network to match demand. If a business requires more phone lines, it’s simply a matter of asking the provider to activate more channels. And if or when demand decreases, trunks can easily be removed with a simple software configuration.


For businesses that rely on ten or more channels of ISDN, switching to SIP can deliver immediate cost savings. SIP channel rentals and calls are typically up to 40% cheaper than traditional ISDNs, and businesses have the extra benefit of consolidating their voice and data services into one single line for further savings.

High quality audio

Over the past few years, bandwidth capabilities have exploded, giving way to high quality speech audio. Depending on the audio codec used, SIP can deliver speech quality that is on par, or better than, ISDN. G.711 codec delivers high-definition digital audio and is recommended whenever bandwidth allows for it. G.729 delivers compressed mobile quality audio and is used when there are bandwidth restrictions.

Make the switch to SIP

Having a thorough understanding of your current tech situation will give you a better idea of how the migration process will carry out. And, asking Merlin the right questions will ensure a smooth transition and give you peace of mind. 


Is your network up for the task?

Merlin will advise you whether your existing routers and switches are up to date. A network survey may be required to check for congestion points that cause latency problems and degrade call quality. We will also ensure your IP PBX or legacy PBX supports SIP.

What are your time constraints?

The process of porting existing numbers and building SIP trunks takes up to 25 working days to complete. This period can be used to educate users about the additional features of the new system and get the workforce up to speed.

How much bandwidth will you need?

SIP requires a steady, high bandwidth internet connection. To calculate your capacity requirements, consider how many simultaneous calls you will need and multiply this by the speed of your chosen codec. G.711 requires around 100 kbps per call, and the compressed G.729 requires around 30 kbps per call.

How business-critical is voice to your organisation?

If your business depends heavily on voice communication, you might want to consider a resilient, high availability solution that will ensure lines remain live in the event of a network failure or emergency. Choosing a provider who can offer robust SIP trunking is a key part of this. However, this can further be strengthened by having both ISDN and SIP trunking operating alongside each other.

Common Questions we get asked.

Is a native or direct connection to your PBX possible?

If this isn’t possible, ensure the provider guarantees their media gateway or session border controller (SBC) will connect with your PBX. A quality provider should have a list of compatible PBX models available that can help you speed up the overall installation process.

Is there a Quality of Service guarantee?

Whereas emails can wait, calls are naturally fleeting. Because of this, it’s important to make sure your provider allows voice to be given priority over other types of data across the network. If you are voice critical or have strong audio quality requirements, then you might want to consider using a dedicated Ethernet connection, Multi-Protocol Label Switching (MPLS) or Virtual Private LAN Service (VPLS), which can support traffic separation to meet specific service level agreements (SLA) on latency and downtime.

What is the provider support like?

Looking out for quality customer service should be an important part of your decision making process. A provider that is knowledgeable and responsive to your requirements will be invaluable, both in the day-to-day running of your telephony and in times of critical need, such as downtime or peak demand.

Can you use third party connections?

Having an internet connection and SIP trunk coming from the same provider can be advantageous, especially if there is ever a need to troubleshoot. However, it’s possible that your business is tied up to an internet provider in a lengthy contract. As such, ensuring the SIP provider can work seamlessly with third party connections is crucial.

Technical SIP Steps

There are a number of ways businesses can adopt SIP, with the most common approach starting with ISDN migration. From there, it’s a simple three-step roadmap to benefitting from a modern telecoms system.

Transfer out the old

The SIP provider takes over existing ISDN lines using a like-for-like transfer. This gives the provider control of the telephony estate and the overall migration process. SIP is configured to the network by connecting the SIP trunk to your IP PBX/PBX gateway.

The SIP PBX sits inside a firewall with a private, non-routable IP address. As Port 5060 needs to be opened, it’s important to use a SIP capable firewall or an Enterprise Session Border Controller (E-SBC) that is designed to properly handle SIP.

Build the SIP trunks

Merlin will put in a port request to the previous ISDN provider. While that happens, trunk are built using dummy numbers and the system is continuously checked. Trunk registration, call quality, two-way audio, code alignment, firewalls and LAN networks are tested for success.

Port numbers

When trunks are ready, the numbers are ported across. Businesses can either use traditional analogue phones with a SIP adopter, or SIP softphones that come built with SIP-specific features. From here, users can instantly start to enjoy the business benefits of SIP.

Merlin SIP Trunks

  • As an established business, Merlin has great experience of deploying and running SIP
  • Around the clock support, 24-7, 365 gives you comfort that recovery from any downtime will be minimal.
  • Merlin offer great technical support, and will have a thorough understanding of your current capabilities and the migration process so a speedy path to full SIP trunking implementation.