I'm kept inside a boxy cage.
A prefix for the modern age,
I make the messages you create
fly at a supersonic rate.
Now that box has me and my friends
inside a box with rounded ends.
You can find us in all your letters,
we make communication better.
But look! One friend is now set free!
The one that's a minute, a second, a year.
Freedom is sweet, but my dear friend.
Alert someone before we're spent!
The answer is a word.
There are some tags that would apply but might be a little spoilery. I'll add them if nobody gets the answer.
Edit: I came up with a couple of hints that continue the story of the poem, so I added the "story" tag. Mind you that it's definitely possible to solve the puzzle just with the poem, but I think this will make it even more... puzzling, which is of course my intention. Here's the first one:
The following day, my friend hadn't returned yet. However, our captor didn't seem too concerned with it. When he approached the box, I took the opportunity to ask him about it:
"Oh, you're so naive, little one", he said. "Your friend didn't escape; I let him go. The plan wouldn't have worked if he had stayed with you all."
"Plan? What plan?", I asked, but he walked away, ignoring me.
Edit 2: Added one tag, removed "enigmatic puzzle".
Edit 3: Hint!
I tried asking our captor again what plan he was referring to. He seemed to be in a more talkative mood:
"You're just bait, that's what you are. We're only using you to capture other, bigger creatures."
I was puzzled. "What kind of creatures?"
"Rodents, insects, reptiles. That's what we're looking for".
"Why not birds?".
He thought for a second, and then said. "We may catch some birds. Parakeets, for example. Those are pretty. But we're not wasting time on pigeons or eagles or storks." He looked at me mockingly. "Anyways, that's more than enough talk for today".
He left again, and I was more confused than ever before.
Edit 4: Another hint!
The title, as lame as it looks, is a clue.
The poem describes something quite literally. Finding out what it describes is the riddle part of the puzzle, but that's not the final answer.