The last scene with Grover and Jane is a flashback. The movie does this often. So, after he leaves the airport because he can't buy an international ticket without a passport, I'm guessing that the last scene was Grover listening to Jane's full message on the voice recorder. Am I wrong?
At least there will be plenty implied.
I'm watching it again right now. I don't think the final answering-machine scene comes last, chronologically. Grover listens to that message before he, Max, and Kate see Otis off to Milwaukee.
Leading up to Grover's decision point at the airport, the answering-machine scene is just one of a series of moments in which Grover drags himself closer to realizing that he misses Jane:
- He watches Max pair up with Kate and drift off in that direction.
- He witnesses the "Cougars" voice a few home truths--lies they've told to each other and to themselves.
- He finally brings himself to listen to Jane's answering-machine message in its entirety.
- He finally asks Chet if he can read Jane's postcard from Prague.
- He watches Otis finally leave for grad school in mid-year.
- Finally, he has an impulse to buy a ticket to visit Jane in Prague. Chronologically, that is the last scene in the movie.
In other words, the "present" scenes run in chronological order, and the "flashback" scenes do, too. Both stories are running forward in time, alternating with each in the same movie. Baumbach lets the past have the last word, following the final "present" scene (Prague impulse at airport) with the final "flashback" scene (Grover and Jane starting their senior-year fling).