Rule 294 - The Highway Code

Railway telephones. If you are driving a large or slow- moving vehicle, a long, low vehicle with a risk of grounding, or herding animals, a train could arrive before you are clear of the crossing. You MUST obey any sign instructing you to use the railway telephone to obtain permission to cross. You MUST also telephone when clear of the crossing if requested to do so.

Laws RTA 1988 sect 36 & TSRGD schedule 9 parts 7 and 8