At the end of the movie Cell we see that Clay has turned into a phone zombie and has become part of the zombie flock around the cell tower. We find out that he was unable to destroy the tower and was not happily reunited with his son.
What I want to know is when exactly did he turn into a zombie? Did he get turned earlier in the movie or did it happen when he walked through the flock around the tower to find his son?
I found this youtube video which clarified that Clay did not turn into a phone zombie earlier in the movie. It happened after he drove the truck to the cell tower, because at the end we can see Clay walking in the flock and the truck is standing there.
Clay became a phoner when he hugged his son at the end and the noise from his son's mouth turned clay into a phoner.
Clay becomes a phoner right when he hugs his son in the literally sense. In the metaphorical sense, the real point of the movie, him and everyone else has always been a phoner.
My other answer that gave a detailed explanation for this was deleted so I apologize for not being allowed to explain it.
