My sysadmin's view of going from Centrex to VoIP (sort of)

November 30, 2023

I recently read j. b. crawford's Centrex, about the telephone service by that name that was offered to organizations in at least the US and Canada. It sparked not so much nostalgia as memories, because up until very recently the University of Toronto's downtown campus used Centrex, complete with what I now know (from the article) is a distinctive Centrex pattern of five-digit dialing for phone numbers inside the organization. In the University of Toronto's case, most phone numbers were 978-xxxx and so it was common for people and documentation to list numbers as, for example, '8-4942', which is what you actually dialed on a Centrex phone. If you were calling from a non-Centrex line, you were expected to remember the '97' in front.

(It's probably not hard to still find online web pages that talk about our numbers that way, never mind physical signs around the university. There are some remarkably old signs on things.)

I'm not sure what drove the university to move away from Centrex (if I was ever told I've forgotten now); it might have been the potential for cost savings, or it might have been Bell Canada suggesting that Centrex rates might or would be going up. The university's general replacement for our Centrex service was (and is) Voice over IP (VoIP). The actual rollout was rather slow, partly because it was just getting started in early 2020 and you can guess what happened next. You can still get hardwired phone lines if you want to, but they're now much more costly than they used to be (and the cost is paid by your department or group), so they tend to be reserved for situations where you really need them.

(We have one such line in our machine room for safety reasons.)

In general I'm sure Voice over IP is fine, but from a sysadmin's view it has a little problem, at least as implemented here at the university. That issue is that it runs over your regular network. There is no separate physical or even logical network for VoIP phones; instead you plunk them down on your department's network in some convenient place (physically and logically), and then they phone home to establish their VoIP circuits. For regular usage this is fine, but one of the times sysadmins may need to use their phones is exactly when the network is broken and they need to call someone about it. If your phone is VoIP and it runs over your broken network, you have a problem.

(The university's current VoIP setup also has some nice conveniences, like emailing you recordings of any voicemail messages people have left you, so you don't have to deal with the phone's voicemail interface and can just play them on your computer.)

The department could have dealt with this problem for us by leaving us on hardwired phone lines even after Centrex service stopped. However, it was cheaper to get us basic cellphones (with voice-only plans). At this point we've gone through three different models for reasons outside the scope of this entry; the current one and the previous one have been actual Android devices, which I haven't been too impressed with. But they work to make phone calls, probably, and they also do a few other things (also).

(One of the things we don't use these phones for is MFA authentication; instead we have much smaller and more convenient physical tokens. Nor do we normally carry them around, especially out of hours; in my group, everyone's smartphones spend all their time on their office desks. We view them almost entirely as a replacement for our old desk phones.)


Comments on this page:

Sounds like your VoIP rollout went well. I got to watch a PBX conversion from the sidelines once. The sales team promised "nothing will change," then the VoIP rollout overwhelmed the network. Core business and customer service were both disrupted.

I think they already had wiring to support physically segmenting the phones (and making them PoE), so they did that to favor robustness. But it was a rough week for them.

By pkern at 2023-12-01 12:28:44:

About Android: you've probably already noticed but it is possible to make use of an Android phone without signing it in to a Google account. But it takes extra effort -- eg. disabling GooglePlayStore among other built-in apps; finding alternatives to default Google apps and side-loading all those app APKs; installing apps from F-Droid, etc.

By cks at 2023-12-05 11:21:24:

One part of the rollout apparently going without problems is that our networks were already 1G with a 10G core, and my impression is that VoIP isn't too high bandwidth. However, another part is probably that we don't have all that many phones and people mostly don't use them. If we have a significant number of people spending a significant time on VoIP phones all at once, I don't know what the bandwidth impact would have been.

(Our network is non-PoE so all of the physical phones had their own power. Reading about it now I see that PoE is up to 1G+ speeds, too, although we'd have had to buy new switches to get that; all of our PoE capable switches are old ones that have 100 Mbit data ports and maybe a 1G uplink.)

Written on 30 November 2023.
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