This is what management time is intended for: managing employees and the business. This includes ensuring safety critical roles are being performed correctly.
There may well be an issue of multicollinearity on use of a personal handset vs an approved method of communications. But when use of either, and particularly a personal handset presents the threat of a major and fatal collision, I'm fine with spending the time distinguishing between the two. As others have noted, this wouldn't last long, and I'm sure that the consensus is that no distractions in the cab are overall better.
Okay, let's provide some enlightenment about how such a report would - realistically - be investigated where I am.
We don't have any process in place for checking mobile phone records for personal phones - quite simply that doesn't happen.
One can trawl through CCTV from stations, however this will be of variable quality and is unlikely to provide anything which will absolutely conclusively prove what the driver was doing. We don't have any download data that will show if PA was being used, although we can find out if radio was being used (note the latter will cost a four-figure sum to download).
Bear in mind the drivers will know all of the above, so whilst there are one or two things that can be done to encourage honesty in an interview, the outcome of the investigation may well depend on what the driver tells you or doesn't tell you.
Whilst all of the above is going on, the driver is stood down so can't perform any useful duties, and management hours are tied up.
It's quite possible, probably likely, that the driver will receive some form of ad-hoc monitoring -- although of course it's not actually possible to see what's going on inside the cab although it might be possible to sit behind the bulkhead and listen. Finding people to carry out monitoring outside of Mon-Fri office hours can also be tricky.
As I say, I'm not for one moment suggesting people shouldn't report things of concern - not at all. Just take a few moments before pressing the 'send button' (so to speak). I can't stress enough that virtually every single report that comes in turns out to be unfounded.