Welcome Back!
UCbuyer Logo

Your Source For Enterprise Communications & Collaboration News & Insights

Making the move from ISDN to SIP: How and Why

Posted by Patrick Lincoln

May 12, 2015

++++++++++++++  Guest Post  ++++++++++++++

Although ISDN has performed well over the years in the area of voice quality, its lack of flexibility, higher prices, and inability to deliver equally high data and video quality have paved the way for the growth of SIP. There has been a lot of excitement over SIP trunking in the last few years, but setting the hype aside here are some things to know about making the shift from ISDN to SIP Trunking.

1. Defining your Business Needs

SIP can bring many benefits to a growing business and the actual switch from ISDN is easier than you might think. SIP_Trunking_BenefitsIt will take some planning and research, but the following eight steps should be sufficient for any size business to navigate the transition. Defining your businesses needs is the first step and to this you need to answer a few basic questions:

  • How many phones numbers do you have?

  • Where do those numbers lead?

  • Are incoming calls queued or directed elsewhere if the line is busy?

  • What kind of voicemail system does your business require?

Take the time to assemble an in-depth diagram of your current network. Clearly defining your businesses’ communication requirements in the early stages will help you fulfil them later on.

2. Defining Areas of Improvement

During the second step you will determine what improvements can be, and should be made to your current procedures and processes. These enhancements may include:

  • Updating disaster contingency plans

  • Modifying your unified communications

  • Reviewing staff mobility

  • Planning for seasonal fluxes of communication needs

Look to see how SIP trunking can be used to greatest efficiency in each area?

3. Planning for Change

In the third step, devise a rigorous plan that tests how each change you make to the network will affect the day-to-day workflow of your business. This entails examining how staff accesses data, how incoming calls are directed, if alternative facilities and emergency procedures are fully functional, and how the phones are physically networked. Create a plan to test staff and systems affected by the switch to SIP.

4. Hardware

The next step is to determine the relevant hardware and equipment you will need based on what you have found in steps 1 to 3. Although analogue phone-sets will function with SIP, it may be worth upgrading to IP phones in order to take advantage of their VoIP features. SIP is going to be placing a heavy load on your bandwidth. It may even be worth investing in assured broadband, or ordering a second connection to insure call quality.

Again, be aware of potential interoperability issues and check your hardware and software for compatibility with your potential SIP trunking service. Most PBXs designed after 2007 will be compatible with SIP, but if there is a lack of interoperability you can either replace it with an updated PBX, or purchase a SIP-ISDN Gateway to connect your SIP trunk with your PBX’s ISDN interface. Also, as you add your phones to the IP network, consider the potential need for additional network ports within the switching system.

5. Choosing a SIP Provider

Once steps 1 through 4 are completed, it is time to select a SIP trunk provider. When choosing a provider, it is essential that they be a good fit for your business. Insure that they are not only able to port all of your current numbers to their service, but that they have solid fraud-prevention and notification measures in place to protect you and your customers.

6. Testing

Step six is the opportunity to put your planning into action. Install the new equipment, get the hardware and software configured, and test it out. Once everything looks good, dust off the test plans you devised in step three and run your new system through a rigorous testing and trials stage.      

7. Porting Numbers

For step seven, port your numbers and don’t forget to take into account the time required to do so. Before you begin the porting process, take a few measures to insure a smooth transition. Firstly, examine your provider’s porting agreement for any potential glitches. Then check that you have all the necessary information at hand for the letter of authorization (LOA) that will be required by your new VoIP provider. Call your competitive local exchange carrier (CLEC), if you are unsure of the exact name and address the numbers are listed under.

Once the porting order is submitted to your new carrier, await a confirmation that the request is being reviewed (FOC). Once you are sure there are no errors or rejections in your porting request, you can move on to the final step in the process.

8. Cancel Current ISDN Service

Step eight is to cancel your current ISDN service and return any rented equipment. Once your numbers have been switched to VoIP, the ISDN lines should stop working, but double check that you are not still being charged by your old provider.

If done correctly, as outlined above, the transition to SIP should be an easy conversion for your business. Although SIP trunking offers greater flexibility and lower operating cost than ISDN, it is important to remember that SIP might not be for everyone. Below is a quick summary of the advantages and potential drawbacks of moving to SIP.

Potential Drawbacks:

Since SIP trunking is dependent on reliable broadband internet, some rural businesses may have to wait before their IP providers can support SIP in their area.

Business should be aware of potential bandwidth issues. Although it is avoidable with leased or assured lines, drop in bandwidth could result in a decrease of quality for phone calls made within the SIP trunk network.

Interoperability is becoming less of an issue, but it is still something to be mindful of when switching from ISDN, especially between PBX’s and SIP trunks providers. Always check your equipment, hardware and software, to insure compatibility.  

Security concerns with SIP include the potential for DoS attacks and phreaking of unsecured PBXs. Thankfully, these security risks can be easily mollified by implementing firewalls, session border controllers, and fraud monitoring systems.  


For starters, you are going to be saving money as you centralise your PSTN access and downgrade the unified communications (UC) resources required to operate your business. PRI and T1 lines will become a thing of the past as SIP eliminates the need for separate data and voice lines. Costly ISDN rentals are removed, and you remove call handoff fees within your own network. Your business will also see less money wasted on blocks of unused channels, as SIP can be customized on a per-channel basis. Saving money, however, is just one of advantages.

Unlike ISDN, phone numbers are not tethered to a set location, giving SIP trunking the benefit of portability. Businesses can automatically divert calls to a separate office location if all the lines at one office location are occupied (even if your offices are located in separate countries), and should your business choose to relocate, your current numbers move with you.

Having too many or too few channels is a familiar problem for most businesses with ISDN. With no limitations on DDI ranges and as many as 2,000 channels on a single connection, SIP eliminates this issue by enabling businesses to instantly scale, up or down, on a per channel basis to reflect their fluctuating needs.

Quality is also going to be improved with SIP as high speed connections provide increased HD voice quality. There are even built in safeguards, such as automatized failover to a secondary path, to insure continuation of service should one location suffer a disruption or network failure.

Take the time to contact a dependable SIP provider and ensure that your company has the information it needs to make an informed decision.

For more information on making the transition from ISDN to SIP download this whitepaper.

About the Author: Patrick Lincoln is founder of Unified Communications Company Solution IP, which he set up in 2006. An authority in the industry, Patrick spent many of his formative years building relationships within the telecoms community in the South West of England. You can connect with Solution IP on Twitter or Facebook.

Topics: Voice, networking, Business Case, Best Practices, SIP

Subscribe to Email Updates

Dig Deeper

New Call-to-action

How To Save Your UC Deployment


Have News to Share?

Fill out the form below and tell us what's happening.  We'll get it online as soon as we can (and you may even hear from one of our journalists).

What's Hot on TheUCBuyer