Did the murder really occur in Censor?

The phone call is partly real, partly imagined: it is a harassing phone call from a member of the public, which Enid transforms (in her mind) into a message from the producer. Note that the "information" that she gets is information she already obtained during her visit to the producer, the call even uses the same wording (e.g. "juicy sequel").

Enid has started to disassociate from reality after killing him:

She is now going deeper into the trauma that resides in her since losing her sister.

From now on the movie also mixes reality and imagination, and offers visual clues for this: a TV showing a scene from the movie Don't Go in the Church morphs through video static into the trees she sees on her way to the place where they're filming the sequel.

From then on the aspect ratio of the movie slowly reduces to 4:3. Then she kills the "Beastman" and she somewhat snaps out of it -- to indicate this we see damaged, scratched film.

What follows next is another somewhat lucid scene where reality seems to get the overhand, until she starts "using the video remote" (note that the picture quality at that moment also switches to "video", indicating her delusional state).

Again the aspect ratio changes to 4:3. From then on she almost completely replaces the events in real life with an imaginary version of them, ending with her driving the actress she "saved" (and which she imagines to be her long-lost sister) to her parents. Her delusion is almost complete at that moment, with reality only invading through short burst of static.

As Looper explains:

As her obsession with Alice grows, though, she loses sight of her goal and morphs into the very person she fears.

So, what made this unstable censor lose control? "Don't Go in the Church," a film about sisterly betrayal, strikes a nerve for Enid. This viewing brings forth years of emotional turmoil over her presumed dead sister. Her need for closure, coupled with the visual similarities between Alice and a police sketch of Nina, drives her to act out.

So yes she really murders all those people. Her mental state was always unstable, even previous to her murdering the producer she experiences periods where she mixes the trauma from her youth with the movies she sees.

Also note that her murdering people fits into the theme of the movie. From the same Looper article:

Coming full-circle, the end of the film includes a radio broadcaster within Enid's fantasy announcing the eradication of video nasties, which drops the crime rate to zero and boosts employment rates. The public is gleefully informed that "there's nothing to be afraid of anymore!" Enid, who bought into the misguided conservative fears, is unable to register that she has done anything wrong. While it isn't shown in the film, it's clear that the public would go on to blame her consumption of morally corrupt films for her crimes without factoring in her mental instability.