The Black Phone got a change in the script by the director Scott Derrickson suddenly.
In the movie, the basement is where the Grabber [villain] holds his victims, there is a black phone that has been disconnected hanging on the wall. Finney can hear the voices of the Grabber's former victims ringing out ceaselessly. One of them is Finney's close friend Robin (Miguel Cazarez Mora).
Since the Grabber abducts Robin, we only get to see him briefly after we first meet him at the opening of the film. And Derrickson understood he would need to make up for that in the conclusion.
“In the script, Robin wasn’t in the room. It was just a phone call,” Derrickson said. “A day or two before we shot that … it suddenly hit me out of nowhere. I was like, ‘Oh, the audience wants to see that kid again. We got to see him again.” The director added.
He continued, “It’s not going feel right if we don’t see him again.’ I was like, ‘Where’s that kid?’ and they were like, ‘We just flew him home.’ I was like, ‘Get him back. You got to fly him back.'”