I shall pontificate briefly. Technical solutions for social problems rarely work well.
A union will defend freedom of association; the capacity of the worker to speak with their union while in the place of work and at all other times. The worker is obliged to do so safely. Exactly what is safe depends on the speed, vehicle, load, road, traffic and environment. In most jurisdictions with unions, an employer is responsible for describing how to work safely, and sometimes must do so with the union.
Accidents can also be intentional, with workers using phones to coordinate via side channel. Blocking the phone won’t solve that scenario, because the workers will switch to sign language, planning, or UHF CB radios and a spoken code.
Best is to ask the workers how they would like their use of phones to be made safe.
It isn’t practical to detect the difference between an idle LTE phone and an LTE phone being used for a conversation. Both generate RF packets. The codecs used can result in irregular packets (silence suppression, background recreation), so it sometimes isn’t practical to prove a voice call is in progress.
If it were me, I’d avoid working on the problem because I’d rather not be the target of revenge, even misplaced revenge. Note that this thread is visible to the general public; no authorisation was needed to see it.