That could be done by a simple switch trigger:
size: 0/0
activation: none/repeatedly
condition: player hasWeapon "sebprc25"
onActivation: # setradiomsg "blabla"
onDeactivation: # setradiomsg "null"
off course you would need to hide the radio message
by default for everyone in your init.sqs, as the deactivation
would not do it's job as long as the condition of the trigger
hasn't been met to activate the trigger before.
:note - remember that the action/script/command, activated
by the radio call, would still run on every machine even if
it's not available/visible to somebody. That's because
radio-calls are set off global. For local use i would suggest
custom actions
~S~ CD