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Make the move from PSTN to SIP Trunks

Make the move from PSTN to SIP Trunks

Once you’ve assessed your options and decided that SIP is the right way to take your business, you’ll want to jump right in. Often, the journey from decision to implementation is a disconnected one. Recognising this, we’ve put together a guide to walk you through what you should expect when kicking off your SIP implementation.


Every SIP implementation needs to begin with a plan. You’re probably used to planning IT changes that often revolve around hardware and downtime. SIP planning is a little different.

The most important part of planning your SIP trunk go-live is validating your number portability. To ensure that no problems arise during the porting process, checking the structure of your current ISDN is crucial. When your new provider places an order to port your numbers from PSTN to SIP, they must provide vital pieces of information before a porting date can be agreed. This information must be correct across historical systems. We often see delays during the process when businesses have moved address or added DDIs in a staggered manner.

It is best practice, but difficult, to keep an audit of all changes to your ISDN infrastructure. If you haven’t done this, it may take some time to get right. Starting this task ahead of time helps with a smooth process transition.


Another part of your planning process should be your communications plan to the rest of your business. Setting expectations when making any business change is important. When the change will impact the way users communicate, directly or indirectly, it’s important to keep users informed.

One suggested method to work out how much information you need to provide is to rank stakeholders. This can easily be achieved by using a power and interest grid. Plot along your x axis how much power a specific user – or group of users has – then plot the y axis as how much interest in your project the users - or group - has. For those users that appear in the top right corner, make sure you constantly and actively manage their expectations. This will likely be the person responsible for the heaviest phone user groups.

For those stakeholders in the bottom left - perhaps those that don’t use the phone and have no influence – these can be ignored. Other users falling into the middle of the grid should be kept on side with regular updates as to how the project is progressing and a timeline of when events and changes might happen.


In the build up to your SIP migration, your provider should run you through key scenario simulations in a test environment. The goal here is to ensure every possible SIP interaction has been tested on your new SIP platform, before your numbers have been moved over in real-time. This is not testing that the SIP technology works, but that your specific SIP partition is configured correctly for your needs.

Scenarios here typically include peer to peer calling both incoming and outgoing, applying call treatment for business continuity scenarios and emergency services. Until SIP testing is fully completed, you shouldn’t proceed with your SIP migration. If a call scenario is not tested or not successfully completed, it cannot be guaranteed to work when the real-time event happens.


When planning and testing is completed, you should have received your porting date. If you haven’t received this by the time you have completed planning and testing, there is no need to worry. If everything preceding the port is in place, it’s just a matter of time before the port commences.

On the day of porting, your users are expecting a change to happen, and your provider is on hand to provide updates as changes occur. Your numbers will start to move from one network to another at the given port time. Dependent on the volume of numbers being ported, this could be a short or long process.

Once all numbers have been transitioned from one network to the other, your ISDN installation will be made redundant by the network. All devices previously connected to your PSTN and ISDN services must be changed over to continue service.


BT Openreach automatically decommissions your PSTN and ISDN services. However, they do not collect any equipment from site – as this cannot be reused. Whilst this appears as just a remote decommission at first, engineers are scheduled at your local telephone exchange to conduct the physical work where disconnection of your circuits occurs.

Ultimately, you don’t need to do anything to decommission your historic phone lines. Your SIP provider should provide you with handover information detailing how to order more SIP trunks or to log a fault should one occur.

Revisit business case

Once your business has adopted SIP trunks and received the first few bills, you can start to reconcile your business case. With any project, the most important part is realising the benefits of the change you have just made. However, it’s also one of the most common areas that gets omitted.

When you receive your first SIP bill, you will immediately realise the cost savings as your monthly ISDN charges reduce to much smaller SIP rental charges. Call charges should have decreased too, even in the first month. The removal of internal calling charges coupled with the general SIP reductions should provide an immediate benefit.

Come the end of the year, it’s important to review the costs saved on a yearly basis. You can use these cost savings to drive the business case for other sites that might need to transition to SIP or push the agenda for further cloud technology.

Outside of cost savings, year end is the right time to review the scalability, quality and flexibility benefits you will have received through your SIP implementation too.

Once your business case is prepared for SIP, and you’re ready to make the move from PSTN to SIP, get in touch with one of our SIP specialists. We’re on hand to start your planning, testing and communications strategies.

For more information, why not book a call with one of our team or reach out to us on live chat.

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