Numbers 10:6

And when you blow an alarm the second time , the camps that are on the south side shall set out . An alarm is to be blown whenever they are to set out .

At the sounding of a second blast, the camps on the south are to set out. The blast will be the signal for setting out.

When ye blow an alarm the second time, then the camps that lie on the south side shall take their journey : they shall blow an alarm for their journeys.