A VHF voice call or broadcast may be preceded by a DSC urgency or safety call. Both calls include the information on VHF channel on which the call will be made. Urgency calls will sound a an audible alarm similar to that used for distress, except it will be a single tone rather than two tone alarm. Safety calls sound an audible tone, not an alarm. VHF radios used by boaters can receive but not send urgency or safety calls.
An urgency call indicates that the calling station has a very urgent message to transmit concerning the safety of a vessel or a person. A severe storm warning is an example of such a message.
A safety call indicates that the calling station has an important navigational or meteorological warning to transmit. Marine weather forecasts and broadcast notice to mariners are examples of such a message.