Should I Replace ISDN with SIP?

What you need to consider

ISDN is an acronym of Integrated Services Digital Network, which can be defined as a series of communication standards that allows the simultaneous digital transmission of data, voice, and other services over the public switched telephone network. The introduction of ISDN technology represented a move from analogue connectivity to digital, which offered significant improvements in quality. Although SIP (session initiation protocol) has been around for several years, it is only recently that advances in technology that have now made SIP an viable alternative to ISDN. The purpose of this article is to explore whether a business should replace ISDN with SIP.


The SIP market has developed extensively in recent years mainly due it the significant improvements seen with broadband speeds, and in particular upstream speeds, as high speed internet is a fundamental component of an stable SIP connected network.

The main barrier for any business that wishes to implement SIP into their organisation is whether or not their public branch exchange (PBX) is SIP enabled. If your PBX is SIP enabled, then there are some extremely compelling benefits in switching from ISDN to SIP. If however, your business doesn’t have a SIP based PBX, then SIP should be something that you explore when your PBX is due to be replaced, as all modern PBX equipment is SIP enabled. Some of the advantages that SIP has over ISDN can are explored below:

Lower call costs

SIP trunk providers are usually able to offer lower call costs than ISDN providers, but this will vary on a case by case basis, and the lowest charges are typically offered to customers delivering the highest call volumes.

Free call routing between sites

It may be of particular interest to businesses that operate from multiple sites to know that almost all SIP providers offer all calls made between these site free of charge.


SIP Trunks are extremely flexible as they cab be scaled up or down to the exact amount of channels that are required by the business. In comparison, provisioning new ISDN lines increase the amount of channels by 10 at a time, which may be a lot more than what is actually needed by a business.

Number portability

One of the most frustrating elements of business telecoms is the fact that if a business is connected to the PSTN via ISDN, then they will be unable to keep the same telephone number if they move outside of their local BT exchange area. If however, the business in question connects to the PSTN via SIP, then the number can continue to be used regardless of the location of the business.

Contact us for further information