This was less than 25 minutes long, but the story is so sad I'd remember this from time to time. It is only topped by that other sad OVA I saw, Fafner in the Azure Right of Left.
Noburo and Mikako Nagamine are close classmates. However, Nagamine was chosen to be a Select Member of the UN Space Army. When Nagamine goes away on a Tarsian survey (Tarsians are aliens who have clashed with humans), her means of communication with Noburo is through her cellphone. They send emails to each other. However, as Nagamine's ship goes further away from Earth, it takes longer and longer for her emails to reach Noburo.
"Voices" accomplishes to convey a lot in only under 25 minutes. At first, Noburo and Nagamine's messages to each other are just plain all about their activities. It gets sadder when comes on board at Lysithea, as it goes further away from Earth. I mean, it must be really hard emotionally and mentally waiting for a message from someone who's no longer on Earth. You're worlds apart, literally. And not knowing if they'd ever come back, but you're still on Earth and living.
The saddest point was when the Lysithea had to make a warp. Nagamine's email would take about 8 years to get to Noburo! So she composes an email that says 'Hello, Noboru, who's become 24 years old! I am the 15 year old Mikako.' It's depressing. And it's actually the last email that she sends. At first I couldn't decide who's life is harder. But I guess Mikako's got the hard part. She's the one fighting aliens after all.
Anyway, the mech designs here are okay. The cockpit is cool though. But I don't like how Mikako is drawn sometimes. It's only less than 25 minutes so I guess no need to do more detailed designs, but why should she be in her school uniform while piloting a mech? But still, nice story.