Most likely, the call has been canceled. This often happens when the first unit arrives at the scene, surveys the situation, and determines that the request for assistance can be handled by that individual unit. Therefore, the other units will then be canceled so they are ready to take another call.
Firefighters will tell you, this cancelation always seems to happen as the big red truck comes up to an intersection and surprises the citizens in their vehicles sitting at the traffic light. It is often times safer to complete the passage of the intersection and then turn off all of the lights and siren rather than turn them off as drivers have already reacted to the apparatus’ presence.