Yes, it happened
To understand why we know this you need to understand the nature of the heptapod language on peoples' perception of time.
No time travel is involved. The alien language breaks the normal human perception that time is linear so people can perceive all of time right now. Hence Louise's premonitions of future events. For her to be able to recall a future event (language is a little confusing when we are talking about cyclic time which might be one of the points of the movie) the event has to be in the timeline in the future.
What is less clear is whether we are seeing the event or her premonition of the event. (see this question and the discussion: Why did Louise forget she had called the Chinese commander?)
Another issue that is raised but not fully resolved in the movie is the issue of choice. It is implied that choices made now can influence the future (by implication this suggests that premonitions are of a possible future not the future). Did the Chinese general have a premonition of the effects of future war and this helped him avoid it in the present (discussed in this question: Why would Louise telling the Chinese commander about his wife's last words change his mind?)?
The movie skips over the implications of this paradox but clearly implies that all visions are of events that will happen and therefore can't be avoided. In the case of the conversation with the general, the reason why Louise knows the general's private number is because of this future conversation so she can't get the knowledge unless the conversation happens in the future.