I'd say as long as there's a means of anyone getting hold of you in an emergency, e.g. a switchboard that could get hold of you within a few minutes, then it's absolutely fine.
There are too many rights and entitlements these days. If you're in paid time then you should be working, so have no need of a pocket distraction device.
But for someone working independently then it's a benefit to the employer. Provided that the work is measurable, i.e. you can somehow check that they're actually doing stuff. E.g. that light bulbs get changed etc. Most employers with vans have trackers, so they could see if you weren't moving or were parked in the pub.