Madhouse's recent OVA, "Arata Naru Sekai" is a one-shot 30 minute OVA without much background. I can gather that:
- time-travel is prevalent in this world
- the 4 girls seemed to have traveled far into the future
- there was perhaps some kind of imminent disaster in the past
- the disaster perhaps happened very soon after their jump into the future
- the past that they lived in was already starting to fall apart
So what happened after the cell phone was destroyed? Was it all a dream? Or did it transport them back to the past or some alternate reality?
EDIT: I found the synopsis at MAL which sort of confirms some of the premise (though I don't really see any evidence for some of the claims in the MAL synopsis), but it doesn't shed any light on the ending.