Throughout the film's second act, the climbers drop their backpack containing their precious water bottles and drone onto a lower communications dish. Hunter volunteers to rappel down to retrieve it. She grabs the bag and is seemingly hauled back up to the platform by Becky after a horrifying slip.
The Major Twist: Several days later, as Becky attempts to use their charged drone to deliver an SOS, she becomes increasingly delirious. When the drone is crushed by a passing truck near the diner below, Becky's sanity breaks. It is then revealed that Hunter never survived the drop to the satellite dish. Hunter slipped, hit the metal dish, and died on impact. The "Hunter" who spent the subsequent days strategizing, talking, and comforting Becky was entirely a hallucination—a coping mechanism conjured by Becky's dehydrated, trauma-stricken subconscious to prevent her from succumbing to despair.
The Ending: Looking down, Becky sees Hunter’s lifeless body rotting on the dish below, being picked at by vultures. Realizing she is utterly alone, Becky endures an attack by a vulture, kills the bird, and eats its raw flesh to regain strength. She climbs down to the satellite dish and realizes cell service is only reachable lower down. Accepting the harsh reality, Becky writes an SOS text to her father, types send, cushions the phone inside Hunter’s opened torso for protection, and pushes the body off the dish. The body crashes to the earth, the signal connects, and the message goes through. Emergency services and her father, James, rush to the desert tower. Becky is brought down safely and tearfully reunites with her father, finally having chosen life.