Numbers - Chapter 10

Moses is told by God to make two silver trumpets.  These trumpets are to be used for various purposes - including "calling the people together."

In Paul's first letter to the Thessalonians (Chapter 4) Paul gives details of the return of Jesus to call his people together:

"For the Lord Himself will descend from heaven with a shout, with the voice of an archangel, and with the trumpet of God. And the dead in Christ will rise first.  Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And thus we shall always be with the Lord."

The silver trumpets in Numbers 10 are a foretelling of the trumpet that we will hear when Jesus gathers his people to himself.