Who is the narrator? Aaron. Undebateable. Who is he calling? Why is he calling? Let's start with which Aaron is calling. It is Aaron(2). Aaron(3) is trapped playing any number of past Aarons, whichever ones he has on his recording.(More about him later.) Aaron(2) is the one who fights with Aaron(3) and decides to exit. He leaves to go to France, which is shown only at the end of the film. (This way, we don't find out about Aaron(3) until the film's grand revelation.)
Aaron(2) calls Abe(1). He does not need to identify himself, Abe knows his voice. Abe listens. He does not interrupt. (Something Aaron could never do.) Aaron is clearly attempting to control Abe. Why would Abe believe that Aaron is a real time traveler? It is due to Abe being gassed, drugged, and locked in a bathroom for at least a day. If Aaron is hoping that Abe will be content to know that he already built a time machine and not go any further than that, he should realize that it is beyond man's ability to retreat so near to such an achievement. Aaron(2) has learned that Abe(2) may have corrupted Abe(1)'s box."He's got it wired wrong. It doesn't work." Abe is attempting to prevent time travel from occurring in the current time line. "You can't watch them forever." Abe(1) will work in secrecy, rather than in the open. Sounds like the beginning of our story, doesn't it?
"They'll be building their own boxes in another day. And yours already knows what they've built. You're not going to be able to watch them forever." Here Aaron is relating that Abe knew it was capable of time travel long before Aaron did. Thus, Abe(2) can not travel back to the moment before time travel occurred to stop it. He can fail-safe, corrupt the box, and delay events; but prevention is out of his control. Abe can only perform damage control. Aaron(2) is indebted to Abe(1) for his creation of the box and he seems instrumental in overseeing its recreation. (Call it a Safety Paradox: One exists and insures their own future existence) Aaron is angry at Abe(2) and recognizes that Abe(2) has already attempted to prevent Aaron(2) from existing. Abe(1) made Aaron(2)'s life possible. The debt has been repaid.
When I first started to discuss this topic on line, I was the only source. Everyone was determined to believe that Aaron(3) was calling Aaron(1). I didn't have the book written back in those early years. When the book was released, more and more fans started to grasp the significance of this detail. Some refused to accept this. Later, while promoting the DVD release of Primer in France, Carruth actually admitted that the narrator was the second Aaron. Asked if he was calling Abe, Shane replied that it could only be Abe. End of discussion.