For a mobile phone offence in an unknown vehicle (not HGV), if the licence holder has a vocational category they will (or should) receive a warning letter via DVLA advising them their actions are considered below those expected. The letter is on behalf of the TC.
Once the letter had been issued further offences are passed to the TC by DVLA and a disciplinary conduct hearing will be likely.
In a 3 month period 1051 drivers received such a letter. In the same period over 200 of those re-offended and were in front of a TC - or so the government public consultation claimed.
If caught in a HGV - this is reported directly to the TC by DVLA and a conduct hearing will follow.
The Senior TC’s statutory document No 6 details the suspension period the driver will receive. If it is apparent the call made in the HGV was work related, there’s a high probability the TM/Operator will be in front of a TC as well