Set the custom radio channel's call sign.
Available special parameters:
- "$KEY" (reference to a localized text)
- "%CHANNEL_LABEL"
- "%UNIT_SIDE"
- "%UNIT_NAME"
- "%UNIT_RANK"
- "%UNIT_ID"
- "%UNIT_REF"
- "%UNIT_GRP_NAME"
- "%UNIT_GRP_LEADER"
- "%UNIT_VEH_NAME"
- "%UNIT_VEH_POSITION"