"Can I borrow the phone?"
Translation:Voinko lainata puhelinta?
Yes, I too thought that "puhelimen" could be correct since it is a single definite limited noun. And I thought that "Voinko lainata puhelinta" would more properly be translated as "Can I borrow a phone," not "the phone." So my answer was marked wrong. So it goes with Finnish!
"Voinko lainata puhelimen?" with the word "puhelin" in the genitive-like accusative doesn't sound very natural (entirely neutral) in a context where you want to borrow it just to call etc. since it emphasises the phone as a (whole) item. You could even translate it as "Could I borrow this entire phone?". "Voinko lainata puhelimen?" could sort of work if you e.g. wanted to take it home, for a longer period of time, with some additions ("Hei, voinko lainata tämän puhelimen kotiin?").
However, you can certainly argue that a good translation for "Voinko lainata puhelinta?" would also be "Could I borrow a phone?". :)