What if you're being reimbursed for your phone? I assume they can only see activity on my logged in GSuite account. Can't imagine that they have access to anything beyond that.
All they do is hand me a certain amount of money every month. The plan is my own, and they didn't install any software on my phone. All I do is run Gmail through the built-in mail and calendar apps on my iPhone. I also have Slack on my iPhone.
I'm not saying that's unreasonable, but I've heard of employees getting caught leaking by communicating through cell plans paid for by the employer.