The poem starts by asking the reader to "Take" telegraph wires and combine them with a moor (an uncultivated and usually rather unforgiving area of land typical of the English countryside).
These wires might transmit actual telegraph signals, but it's just as likely that the speaker is talking about telephone conversations; when telephones came along they sometimes repurposed the older technology. In any case, these wires are sending signals over a big, empty, wild plot of land.
Without the wires overhead directing conversations this way and that, the moor is a quiet, "lonely" place. The arrival of this new technology, however, extends communication (and, implicitly, community) across this desolate moor, making it metaphorically "come[] alive in your ear" (in the sense that you can now pick up the phone to listen to the message transmitted across these wires).
The caesura after "together" creates a dramatic pause before the following sentence: "The thing comes alive in your ear"—the new creature, the telephone system, suddenly lives! These opening lines sound like a science experiment, one that speaks to both the wonders and weirdness of technology: "take this, mix it with that, and bam: it comes alive." Readers might even think of the famous "He's alive! Alive!" line from the 1931 film Frankenstein.
On that note, this notion of being alive subtly implies that human beings can't fully control the things they create: technologies take on a life and logic of their own.