Aravinda, I'll start with your question in terms of some of the cost categories. I've touched on them earlier on the call. Just to reiterate, I put them probably for the wireless side into 3 categories. And I wouldn't underestimate the impact that installment plans and the related margin improvement has had. If you were to look at our 370 basis point margin expansion in wireless, about 1/3 of that comes from hardware margins. And so that shift has been a good one for us and a good one for the industry in Canada. The second piece relates to channel mix and costs on channel mix have just become much more efficient, especially as we move to direct channels and digital channels. And that's coming through. And as we said, our COA and our COR and even as volumes improve, it's a per unit cost that has actually come down quite substantially. And so that's been helpful.
And then the third is what we lump together as back office costs, including our call centers. And so there are a number of factors in there, but if you took even just the call centers with the migration of our base more and more to unlimited, we are seeing the reduction in call volumes, for example, that we had expected. And so that continues to drive it down. In terms of the sustainability, other than the first one, channel mix and back office costs, we continue to see opportunity, and they are sustainable.
And similarly on hardware margins, although those are more impacted by market conditions. And we'll continue to follow sort of how that plays out in the market. But we do believe, it is an opportunity for continued improvements. And then on the cable side, we're seeing much of the same in terms of categories. I'd replace hardware margin with content costs, which in our cable business represents 50% of our cost structure. And what we've seen is, with the Ignite platform, more of an ability to change the packaging and channel lineup so that we are dropping channels that cost us money, but aren't of interest to customers. It sounds very basic. But the team has been very particular in going through that and reducing costs. Even in the event that it's a 1%, 2% or 5% cost reduction on an $800 million cost base, it ends up being significant savings. And then finally, self-install and reduction of service truck rules, some of that is capitalized, but some of it is OpEx. And so you're seeing that play out in both our capital improvements as well as our OpEx. We continue to see opportunities to continue to improve that. And so again, I would put those in the category of very much being sustainable.