Him starting to tell their specific stories what JBL did to those callers... That would be hearsay as he wasn't there for those interactions. But he hasn't done that. Maybe he will later, maybe he won't.

"The rest were just thanking me because they went through the same thing."-Meltzer reporting what JBL did to those callers

Meltzer is reporting that these people say they went through the same thing, they have not came out and said they went through the same thing, the only evidence that this happened is Meltzers report of another person's words.

The definition of Hearsay-"the report of another person's words by a witness, which is usually disallowed as evidence in a court of law." and "information received from other people which cannot be substantiated; rumour"

Until these 15 people come forward and confirm that they went through the same thing as Meltzer has said then this is literally the definition of hearsay as the information (they went through the same thing) received from other people (the callers) has not been substantiated (we only have meltzers word that they have said this).

Nobody is disputing that meltzer received a phonecall at some point, it's taking Meltzers reporting of the contents of the call as fact and evidence instead of hearsay that is the problem.