My general interpretation of the movie is that the future is immutable. I realize this is a point of contention for some, and of course generating that sort of philosophical discourse it ultimately the purpose of science fiction IMHO, but this is the side of the fence that I sit on.
The idea is that all of time is determined in advance, and that our perception of time from a fixed point ("the present") is what leads us to believe things "haven't happened yet" and "can be changed." The heptapods perceive time differently. They see all of time at once (at least within the scope of their own lifespan and experiences) and learning their language somehow changes our own perspective to do the same.
Would the 18 months later conversation between Louise and General Shang in Arrival still be the same after Louise having accessed the information from that conversation before she "physically" reaches that point in time? If not, wouldn't that create a paradox?
Working under the immutable time theory, of course the conversation would be the same. It would need to be, because everything it already decided. Therefore, no paradox is possible.
For the immutable time theory to work, i.e. Louise to be unaware of her calling him, she shouldn't have that information which after the events of the movie she does.
I have my own theory about why Louise appeared to be surprised during her future conversation with the general. It's not that she was unaware or had somehow forgotten anything - her suprised expression was a kind of "soliloquy" for the benefit of the audience, to keep the mood of the scene going, it was not "in character" shock.
That would support heptapods trying to change their fate by transcending humans and asking for their help.
We don't actually know that the heptapods came here in an attempt to change the future. They could have foreseen that humanity would save them from their crisis just as easily as they foresaw the crisis itself, and they only came here to contact us because - again - they foresaw that this is when they first made contact with us.
it challenges their ability to be able to perceive the future, as there would be infinite possibilities and also they would have choice to change their fate which the movie plot goes against.
If you prefer the multiple timeline theory, the most obvious explanation for this issue is that you see the current future assuming nothing changes. Every time you "remember" a future event, it pertains to the timeline you're currently in. In essence, the future you see is not guaranteed to come true, however, you retain the "memory" of that event even though it won't happen anymore. Because once you've "remembered" it, the "memory" exists in your present mind.
This handily avoids any paradox for Louise's future conversation with the general. At the moment she made the phone call, there was a future meeting with the general where he gave her the information she needed. No matter how the future may change before that meeting actually happens, it doesn't change the information she has in her mind due to having perceived something of the future.
Like I said, I prefer the "immutable time" interpretation. =P