[Vespasian]

If the employer provides a work phone for that purpose which may be turned off unless the employee is working, I have less of a problem with it.

It's the offloading of cost and the (potential) encroachment into the employees private life that seems most problematic.

Aside from this it's the usual paranoia that some employers always had more then enough of. The company may or may not benefit from this kind of tracking and ultimately only legislative action / worker representation (unions) can limit the extent.