SIP vs ISDN – Is the Battle Over?

Recent figures from analysts Illume show that the SIP market grew in the first half of this year by almost 15% whilst ISDN connections continue to decline.  So is it a foregone conclusion as to which one a business should use?

Looking at SIP providers, the majority promote SIP as being cheaper to install and rent, and with cheaper calls.  Do these claims stand up to a bit of scrutiny?

Certainly the cost of a SIP channel on its own is a lot less than an ISDN channel, typically £5 v. £13.  But this ignores the fact that SIP needs something there to be carried over.   It is fine if you have space capacity on your data connections but, if not, something needs to be provided.  If FTTC broadband exists then, given that this could carry up to 200 concurrent calls, it will also be a cheaper option.   Although many SIP providers omit to mention that broadband fix times are a lot longer than those for ISDN. However we are now seeing FTTC with Service Level Agreements from some suppliers albeit at a price premium.

If there is no FTTC, which is the case in many cities, particularly in central London, then customers will need to install EFM or larger circuits.  Having looked at the cost of putting in EFM circuits, we found the breakeven point at a central location in London was nearly 60 channels.  In Colchester it was nearly 100 channels.   That would probably be sufficient for companies with 300 employees.  So for smaller companies it would be cheaper to put in ISDN.

So what else does SIP offer?  Certainly it enables you to keep your numbers if you are moving offices and changing exchanges.  Business continuity is certainly better. In the event of a line failure SIP numbers can be diverted almost instantly for each DDI on an individual basis, whereas ISDN30 can take up to 24 hours and then only to one number.

Hardly any suppliers discuss the fact that SIP can be used as a means of rationalising a mix or ageing PBX base, and that this can deliver major reductions in maintenance and other support costs.

So yes, SIP can offer something – but not in every case. We offer free independent advice, so if you are not sure which is the right approach for your business call us on 020 8912 0845.


EFM – Ethernet First Mile

FTTC – Fibre to the Cabinet

ISDN – Integrated Services Digital Network

SIP – Session Initiation Protocol