How could Louise be able to tell the Chinese General his wife's last words on the phone in the present while he told her that in the future?
(Unmasked spoilers, for those who care)
This is a fairly pivotal plot point in the movie - learning the Heptapod language somehow gives you the ability to see the future - in the form of memories that haven't happened yet. It "changes the way you experience time."
The basis for how the language works is not clearly explained. Louise says something about how "learning a new language affects the way you think." This is true from a certain point of view - as a simple example, in English you "make a decision" but in French you "take a decision." People who speak French might think of decisions as possessions, while people who speak English think of them as actions. The Heptapod language is obviously an exaggerated case - changing the way you think to the point that you can experience time in a non-linear fashion.
So, as Louise was learning the Heptapod language by communicating with them, she was slowly beginning to have memories of the future. It started in her dreams - mainly of her yet unborn daughter - but at the climax of the movie, she "remembered" the conversation she had (will have) with the general in the future while she was on the phone with him in the present, thus allowing her to convince him to stand down his attack.