I think most of us are in the situation that our employers don’t explicitly want us to have Slack/Teams on our phones. They want us to be available.
Slack/Teams on my (personal) phone means I can run an errand in the middle of the day and still be available. I’m happy to use my personal device for it. The alternative is having much less flexibility.
If my employer expected me to be available outside office hours or when not at my computer it would be a completely different story. Like if I was on call. Then I’d demand they pay for my smartphone too.
As someone who worked before there were smartphones--indeed, before there were mainstream cellphones and laptops--I'm acutely aware of just how "chained to your desk" you used to be in that, if you weren't there, you couldn't be reached. Of course, you were sometimes in meetings. But, for the most part, you really needed to be sitting at your desk most of the day if only because someone might call you with a question. (Yes, a sales rep calling me on the phone as a product manager was the norm.)
Slack/Teams on my (personal) phone means I can run an errand in the middle of the day and still be available. I’m happy to use my personal device for it. The alternative is having much less flexibility.
If my employer expected me to be available outside office hours or when not at my computer it would be a completely different story. Like if I was on call. Then I’d demand they pay for my smartphone too.