Blog | What’s the difference between ISDN and SIP Trunks

Most businesses rely on their telephone lines in order to continue operating, but choosing the right one is far from easy!


In the last few years there’s been a noticeable drop in businesses choosing ISDN (the Integrated Services digital Network) as their go-to phone line. In fact over 300,000 ISDN channels were lost in 2015 alone! But why are so many businesses switching to SIP? Well in 2015, BT announced the service dead with a shut off date of 2025. Although it’s still 5 years away ISDN users need to come up with a strategy to avoid having their phone lines cut when the service comes to its final breath. Thankfully we’re here to help you learn about the difference between ISDN lines and its replacement, SIP trunks.

What is ISDN?

There are two types of ISDN lines and choosing which one depends greatly on the size of your company. If you’re a smaller business that only requires between two and eight voice connections simultaneously, ISDN2 lines would be the preferred option. This circuit gives businesses two speech channels that can be upgraded to four with the addition of another ISDN circuit etc. If a business needs to use more than eight voice connections simultaneously they need an ISDN30 circuit. Like ISDN2, businesses can add additional ISDN30s when they need to use more than 30 voice connections simultaneously.

ISDN phone lines are a professional alternative to traditional analogue lines that benefit businesses by combining both analogue and end-to-end digital data over the same network instantly. The use of digital data enhances the signal giving the user better reception quality – from a professional perspective; high quality phone calls give users the impression of a high quality service!

Small companies can’t afford to experience any hold-ups during the installation process as it could cost them a small fortune to turn their phone lines off whilst it’s being switched. This isn’t the case when setting up ISDN. Installation is quick and easy as it runs over your existing copper cables (used for your old analogue lines).

ISDN lines are scalable too. This means you can add more ISDN phone lines to improve your bandwidth further. Not only will this help you transfer data more efficiently, but multiple lines enable you to multi-task. Use one telephone line to transfer company data whilst the other is used for calls!

The benefits of ISDN don’t end there. DDI (Direct Dial Inward) numbers enable businesses to associate additional numbers with your main one for your ISDN circuit. Your business can allocate different numbers for different handset users (or departments) as each telephone extension can have an individual number. ISDN makes call diversions seamless. When a call comes in on channel one of your ISDN2, it can be redirected out of the office to an external number on channel two. Simple.

Unfortunately the benefits of ISDN won’t be much use when all lines are cut off once and for all. But don’t panic, SIP will provide your business with all the benefits of ISDN and more.

What are SIP Trunks?

The modern alternative to business telephony, SIP trunking is set to overtake ISDN, but what is it? SIP trunking (or SIP trunks for short) is a public IP telephony service that’s based on session initiated protocol lines. It sets the bar for other telephony services as it provides a more economical option to businesses by merging voice and data.

Swapping to SIP couldn’t be easier. We work hard to help companies with traditional and ISDN telephone systems connect to the new public IP switched telephone networks so that they’ll gain the benefits of SIP trunks immediately. We can also port your telephone numbers over to our next generation PSTN (public switched telephone network) voice switches so that you don’t have to change telephone numbers! Find out more about switching from ISDN to SIP here.

SIP is delivered over data lines that are used for internet and Wide Area Network (WAN) connections – which you’ve more than likely already got. By merging voice as another application on these data lines, the existing analogue or ISDN lines are no longer needed. This means that all internal calls stay inside the company’s WAN, avoiding handoffs by the telecom provider; call charges for traversing to the PSTN are avoided. Furthermore, PRIs (Primary rate Interfaces) don’t need to be purchased for every site (like they do for ISDN lines) as calls aren’t handed off to the PSTN; instead they go through the company’s WAN and are handed off that way. With the combined benefit of avoiding call charges and no need to purchase PRIs, SIP can save you up to 50% in rental costs and up to 25% in call charges!

Moving office is a nightmare when you have ISDN; it’s almost guaranteed that you’ll lose your telephone number as it’s tied into a physical telephone exchange (making it a geographical number). Although you might be able to forward your calls, the build-up of costs for continuously renting numbers and lines would go through the roof. This isn’t the case with SIP. Geographical number porting removes the need to forward calls as numbers are hosted in a central database and can be made to ring from anywhere.

When a customer calls your company, SIP checks your availability to get to the phone. If you aren’t available the incoming call will be switched to the secondary calling route (anything from a mobile phone to an alternative office abroad!). The reason for this is that your number will be programmed to ring an IP address on your internal connection and connect to your telephone system. Automatic failover between multiple locations means you’ll never miss a call. If one site is extremely busy, calls will be redirected to the other (not-so-busy) site.

Although you can scale ISDN up to meet the number of channels your business needs, you might have 31 channels and need two ISDN30 circuits. This means that you’re paying for a lot of unused overhead (29 channels). SIP on the other hand is deployed across an existing WAN meaning channels can be added or subtracted to meet business needs within minutes – thus removing unnecessary overhead. The only con of using SIP for mass voice channels is that you need good bandwidth. With 100kbps of bandwidth being used per SIP trunk, you should consider your internet speed first.


Overall both telephony options are great for business, it’s just a shame ISDN is being shut off in the next few years. Although you may not have had any problems with ISDN, SIP needs some consideration especially with the added benefits and changes to business communications in the near future.

If you want to upgrade your business’ ISDN line to SIP, simply get in touch and our team will be able to help you out. Email or give us a call on 0333 313 5000. Alternatively, fill out the form on this page and one of our friendly team members will be in touch…

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