Your old beige phone at home that plugs into a BT socket is an analogue phone. You might have a second generation phone system at work that connects to ISDN - this is generally considered a "digital" system. This was good in its time (1990 to about 2005 I would say), but is now superceded by Internet based phone systems. And these use SIP.
SIP is the technology behind Internet phones. And it's fantastic. SIP gives you almost unlimited lines (depending on your connection to the Internet). Certainly a lot more than the 2 lines that come as standard with ISDN. You get higher quality sound than ISDN. You get cheaper calls than ISDN. And, come 2025, you won't be able to order ISDN at all, because BT Openreach have already indicated that this technology is being phased out.
So what's the downside, and why doesn't everyone just move to SIP/Internet based phones now? Well, the downside is reliability. Let's be clear about this - SIP isn't as reliable as ISDN. At least, not currently. And what I mean by that, isn't that the protocol or the technology isn't as reliable as ISDN - actually, of course, it is - probably more so. Rather, the infrastructure on which SIP operates tends to be less reliable than an ISDN based solution. And why is this?
Well, primarily it's because your Internet based phone system relies on (surprise!) an Internet connection. And if that Internet connection is either a/ down altogether or b/ being slow and a bit rubbish, then your calls are going to suffer. On top of that, the SIP providers in the market are still developing their systems. It's fair to say that, even 10 years on, most SIP trunk and SIP hosting providers are still working out how to deliver a 100 percent reliable service. And they haven't got it right yet. Over the years, we have used services from Gradwell, Spitfire, and most recently TTB (TalkTalk Business) - and none of them have shown the ability to provide a bulletproof service 100 percent of the time. The technology is maturing, and no doubt their offering will get better and better - but right now, do any of the providers offer 99.99% uptime? Well, none that I know of. Finally, the onsite equipment is also often more complex. A modern Internet based phone system relies on IT equipment locally - sometimes a small server (if the system has on onsite - also called "on premises" element), but also the switch, the DHCP server, the router… all of these bits of IT equipment have to be fully functional for the phones to work. The older analogue and digital PBXs in businesses were highly reliable bits of kit - limited, and expensive, but highly reliable - mainly because they are simple solid state devices, generally only configured once or reconfigured by an onsite engineer a few times in their life, and left to hum away doing their single job bolted to the wall for 10-20 years.
Put together then, a SIP/Internet based phone system is not going to be as reliable as an ISDN based phone system.
The other aspect to this is customer expectations. When I was a kid, you would occasionally make a phone call from the house number and it just didn't work, or perhaps you would end up on the same conversation as someone else. Other survivors from the 70s and 80s will recall this as well I'm sure. The system wasn't 100 percent reliable. But over the years, it became incredibly dependable, and we all took the home phone for granted. It always worked, inbound and outbound, the quality was fine, and unless the cable outside rotted away or got cut by accident, you generally didn't get many faults. ISDN in business tends to be even more reliable than that. So customers are used to their phones just working. As an IT supplier I find it striking how much a customer's expectation of their phones is so much higher than their IT. If their computer goes slow, or their emails take a while to send, or even if their laptop restarts itself - they will probably shrug, perhaps huff a bit, then probably just get on with it - because we generally expect that IT will be a bit glitchy. But if there's a glitch in the phone system, the customer tends to react very differently, and they get very frustrated very quickly - because their expectations are so different.
This doesn't apply to mobile phones of course; we are all used to these occasionally randomly dropping, not always getting a line out, or a line in. But we understand this - it's a mobile system, and subject to distance and signal strength, and interference, and all that.
So the question really becomes: How less reliable is the SIP/Internet phone system compared to an ISDN system, and how much of a problem is this?
Well, in actual fact, the difference in reliability overall is there, but it is marginal. And it entirely depends on the build of the system onsite, the quality of the handsets, the quality of the onsite PBX (if there is one), the quality of the local network, the quality of the Internet connection, and the quality of the service provider. Get all of these as high as you can, and your SIP/Internet based phone system will be almost as reliable as your ISDN system. And when I say almost, I mean 99.x percent reliable, or even 99.9x percent reliable. In the last year, with TTB (our current wholesale provider), there have been 2 significant outages and disruptions. The most recent wasn't a complete "down", but rather a degradation in call quality. Overall I would say there was no more than 4 hours of disruption over the last year. That's about 99.95 percent reliability. Does that means customers are happy with the 4 hours of disruption? No! But is this a good record for what is an IT based system? Yes! And then there's the Internet connection. Do these go down? Yes. At Dental IT, we always recommend having a second broadband connection as a failover. So if the connection that the phones use goes down, we can switch to backup. Finally, does the onsite equipment fail? Sadly, yes, from time to time this does fail, like all IT equipment. In this case, we arrange to divert the calls to a landline or mobile number, or another site, until the equipment is fixed or replaced.
So whilst SIP is not as reliable as ISDN, we can make it almost as reliable as ISDN. And does the business benefit justify the difference? I would give an emphatic yes… installing ISDN in 2017 seems as old fashioned to me, as installing a fax machine. The technology isn't dead, but it is dying, and investing in ISDN and ISDN based equipment barely 7 years before it starts getting turned off, seems a pretty poor business decision to me.
Last edited: 29 November 2017