The reference to a trumpet reminds the reader of how the Lord God announced his coming on Mount Sinai (see Exodus 19:16, Exodus 19:19; Exodus 20:18), and so summoned the people to prepare themselves to meet God. In this instance it is John who is summoned. The loud voice John hears is comparable to a trumpet.
10 I was in the Spirit on the Lord’s day, and I heard behind me a loud voice like a trumpet