If someone somehow managed to get their hands on my smartphone (without my knowledge) for long enough to download and install an app, I'd be furious.
I'd immediately delete the app and report them to management for attempting to steal and sabotage my personal property.
If I was a customer of that company, I would cease my business relationships
with that company immediately unless they immediately fired the offending party.
If I was an employee of the company, I'd start looking for another job where my personal property is respected.
Well, they can if you allow it, but I would definitely not allow it. When the company starts paying my phone bill, they can do what they like with my phone, until then, no.
I'd go a step further, actually. Even if they did offer to pay my phone bill, I'd decline. If they want to issue me with a company-owned phone for me to use for company-related tasks, I'd be fine with that, but my personal phone is my personal phone and I will not do my employer's business on my personal phone.