I believe that, as the vehicles do not transmit directly on VHF, (their transmissions are re-transmitted by ATC) they do not, technically, need aviation RT Licences.
I recall that there was an ongoing problem associated with the Schmidt voice switch that produced a 'bong' sound on the Tower frequency whenever a vehicle transmitted, that was identical to some aircraft cockpit master caution warnings, and so it was disabled. I guess they have fixed it.
MJ
