Session Initiation Protocol, better known as SIP, is the most common protocol used to deliver VoIP solutions. SIP trunks are digital telephone lines that effectively route your phone calls over the internet as opposed to over the legacy ISDN/PSTN fixed line network.

The perfect replacement for traditional phone lines

SIP is deployed in on-premise/private cloud solutions to replace ISDN or analogue trunks.  You’ll need an internet connection and a session border controller, which acts as a firewall between the outside world and your phone system.

It is also used to route calls in the background of cloud/hosted systems.  With this deployment model, you won’t see a cost per trunk, but SIP is how calls will be delivered.

Using the internet to route voice traffic is much more cost-effective than landlines or ISDN.  For that reason, regardless of the deployment model you choose, your monthly bills will be lower both in terms of line rental and call costs.

Scalability and Flexibility

With no actual physical hardware involved, it’s easy to adjust the number of trunks you have at any one time. Plan ahead for growth, or for a known peak call period. If you’re planning an office move you can keep your existing numbers, regardless of your destination.


Blackstar monitors your SIP trunks to help prevent toll fraud. We’ll work with you to put call caps in place. You’ll receive an alert at a certain percentage of that cap. Should the cap be exceeded, the SIP trunks will be disabled to prevent any in-progress hack.


If you have an incident that’s impacting your business operations, you can minimise that impact by diverting calls to another site, users’ mobiles, or even a redundant SIP connection. If they’re not already, SIP trunks should be a critical part of your disaster recovery plan.

Benefits of SIP

Because SIP 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, improve resilience, and put in place flexible inbound call routing (i.e. to mobiles or alternate office sites) so that calls are never missed.

SIP’s virtual nature means it is easy to scale the network to match demand. If a business requires more or fewer phone lines, it’s simply a matter of asking us to activate more or remove channels. 

For businesses that still rely on ISDN, switching to SIP can deliver immediate cost savings. SIP channel rentals and calls are typically at least 40% cheaper than traditional ISDN lines.

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 with, or better than, ISDN. 

Man using call recording in a contact centre

ISDN Switch Off

You may have heard BT is planning on switching off the landline ISDN network in 2025. With landlines becoming redundant, you’ll want to think about switching to SIP or to a cloud/hosted solution.

There is plenty of time to plan a move to SIP. Blackstar will work with you to ensure you have the right connectivity in place, provision your session border controller, and to get your numbers moved across onto your new SIP trunks.

The ISDN switch off presents a great opportunity to audit and consolidate your phone system. Moving to SIP will save your business money on both monthly line rental and call costs.

SIP Trunk Call Manager

SIP Trunk Call Manager is an optional add-on to your SIP trunks. It provides centralised inbound call management, allowing you to be fully in control should an incident occur.

Managed from a web portal or mobile app, STCM allows you to change the inbound call routing of your entire number estate at the push of a button, minimising downtime and other impacts an outage may have on your business.

With a host of features, including forwarding to hunt groups comprised of mobile numbers, queuing on network and time of day routing, STCM turns disaster recovery into business as usual.

International SIP

International SIP is designed to support our UK customers who are also operating overseas. We can either provide new local (in-country) phone numbers or port your existing numbers to our service, allowing us to support and bill everything under one roof.

If you have international business operations, implementing an integrated voice service between offices can increase business benefits and also save you money. 

Some clients use international SIP to reduce international calling charges, others use it so that they can present a ‘local’ phone number when calling into certain territories.

A typical deployment would allow you to connect your UK operations to international destinations as required.

Our SIP Partners

We work with a range of carefully selected partners to provide our clients with choice and a solution that meets their communications needs. 

Our close relationship industry-leading technology partners provide our clients with solutions that enhance their business.

Client Testimonial

“The technical team at Blackstar are fantastic. They have gone above and beyond, helping us with out of hours issues when they didn’t have to. Our voice services, mobile devices and internet connectivity are all supported by Blackstar, which makes management so much easier now it is all ‘under one roof’. They are always so helpful and receptive: I cannot fault the service we have received from Blackstar and wouldn’t hesitate to recommend them.”
Paul Targett
Haseltine Lake LLP

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