In Episode 9 of Steins;Gate, why did Feris's message affect the past? Whose phone was connected to the microwave? Who did her message reach to change it, and if she kept it a secret, how did they know whose phone to use on the microwave? Am I forgetting/missing something?
The mistake you're making is that the phone attached to the microwave is irrelevant—any phone is okay.

The phone that matters is the phone that the microwave phone sends the message to.

The message is sent to Faris's father, Yukitaka Akiha.

The reason could be classed as spoilers, so make sure you watch the show first.


   Yukitaka died in a plane crash several years before the events of Steins;Gate. He was the only victim of the crash. However, his daughter later uses a D-Mail to save his life, by making him believe she was kidnapped and forcing him to take the bullet train instead.