SIP Trunks vs ISDN: Which is Best for your Business?

Posted on: 2017-01-06 | Categories: SIP

Phones are the lifeline of your company, connecting your employees internally and externally. It is what you use to communicate with customers, suppliers, partners, regulators and the outside world at large. There is a wide variety of phone systems available out there and selecting one for your business is a big decision. If your choice is not the right fit for the organization, it may turn out to be a costly mistake.

At this point most businesses have two choices for connecting to the PSTN – use ISDN lines or SIP trunks. Each system has its own pros and cons, so let’s take a closer look at each one.

ISDN Lines and Its Benefits

Integrated Services Digital Network or ISDN lines are an enterprise level upgrade to traditional copper lines. Businesses have more complex requirements than your average household and ISDN lines have the capabilities to support them. ISDN lines combine both analog and digital data which can enhance the reception quality. There is a significant difference in call quality when you upgrade to ISDN and it helps in creating a good first impression for customers.

ISDN lines are scalable to a certain extent. So if you have a congested phone system with too many calls at a time, you can simply add more lines to handle the traffic. If you have multiple ISDN lines, you can distribute traffic among them so that no single one reaches 100% utilization. You can also use one line for transferring data while the other one handles voice calls.

Installing ISDN lines doesn’t take much time either. This is vitally important to businesses that cannot afford to have any downtime. Since it uses the existing copper lines, you only need to purchase and install PRI equipment at each office location. ISDN also supports features like Direct Inward Dial (DID) so you can assign a large number of extensions to a single line.

SIP Trunks and Its Benefits

SIP trunking is based on IP technology and is fundamentally different to most other telephony systems. It is built on the SIP protocol and uses the Internet and data networks to transmit voice calls. SIP trunking completely eliminates copper lines. You can use the same data network to transmit all types of media – pictures, text, voice calls, video calls, documents etc. This leaves you with just a single network to maintain and troubleshoot in case of problems.

Since SIP trunks use your existing data network for routing voice calls, you don’t have to connect to the PSTN at any point for internal calls. It means when one employee calls another within your company, the call is not charged for by the phone operator. It doesn’t matter where users are sitting, calls are still free. Charges for external and long-distance calls are also much lower than ISDN lines. Overall, expect to save anywhere from 25% to 50% on your phone bills with SIP trunks.

With SIP trunks, your phone numbers are not tied to a physical exchange or location. This means you don’t have to worry about losing your numbers when you move. It doesn’t matter if you reassign employees between different offices either. You can program phone numbers to ring different devices simultaneously or consecutively, regardless of their actual location.

While you can scale with ISDN lines, you still need to do so in bundles. So if one line can handle 23 concurrent calls and you need 26, you have no choice but to pay for 2 ISDN lines. SIP trunks don’t come in such bundles, you can simply increase bandwidth and add as many channels as you need. You can also remove channels when you’re no longer using them which translates into quite a bit of savings.

ISDN lines are isolated silos which cannot share data.  But SIP trunks are interoperable with other enterprise systems like CRM and ERP solutions. SIP trunking providers are also continuously innovating and adding advanced features for their customers. Features that you cannot implement over ISDN lines


More and more companies are switching over to SIP trunks and giving up their ISDN lines.  Why the rapid migration away from traditional ISDN solutions? Well SIP trunks are set to be the worldwide standard for communications within a few years. They offer many advantages over ISDN lines including significant cost reductions.

Another reason is that telephony operators in some countries have announced a cut off date for ISDN lines. This means they will not be offering those services for enterprises any longer. The cutoff date is usually 2025 or later but a decade is not a long time for companies that have to switch technologies. If you do choose ISDN, you will eventually have to upgrade to SIP trunking anyway.