L'uccello dalle piume di cristallo

Sam Dalmas (Tony Musante) is an American writer living in Rome with his model girlfriend Giulia (Suzy Kendall). Suffering from writer's block, Sam is on the verge of returning to America, but witnesses the attack of a woman in an art gallery by a mysterious black-gloved assailant dressed in a raincoat.
Attempting to reach her, Sam is trapped between two mechanically-operated glass doors and can only watch as the villain makes his escape. The woman, Monica Ranieri (Eva Renzi), the wife of the gallery's owner, Alberto Ranieri (Umberto Raho), survives the attack and the local police confiscates Sam's passport to stop him from leaving the country; the assailant is believed to be a serial killer who is killing young women across the city, and Sam is an important witness.
Sam is haunted by what he saw that night, feeling sure that some vital clue is evading him, and he decides to help Inspector Morosini (Enrico Maria Salerno) in his investigation. He interviews the pimp of a murdered prostitute and visits a shop where another of the murdered women worked. There, he finds that the last thing she sold on the day she was murdered was a painting of a stark landscape featuring a man in a raincoat murdering a young woman. He visits the artist, but finds only another dead end. As he makes his way back to his apartment, Giulia is attacked by the same black-gloved figure, but Sam arrives home just in time to save her and the assailant escapes.
Sam starts to receive menacing phone calls from the killer, from which the police manage to isolate an odd cricketing noise in the background, which is later revealed to be the call of a rare breed of bird from Siberia, called "The Bird with Crystal Plumage" due to the diaphanous glint of its feathers. This proves important since the only one of its kind in Rome is kept in the Italian capital's zoo, allowing Sam and the police to identify the killer's abode. There they once again find Monica Ranieri, this time struggling with her husband, Alberto, who is wielding a knife. After a short struggle, Alberto is dropped from six stories onto a concrete sidewalk below. As he dies, he confesses to the murders and tells them he loves his wife.
Finding that Giulia and Monica have run off, Sam goes after them, eventually coming to a darkened building. There he finds his friend Garullo (Gildo Di Marco) murdered and Giulia bound, gagged and wounded. The assailant emerges and is revealed as Monica Ranieri. Sam suddenly realises that he didn't actually miss anything during the first attack; he simply misinterpreted what he saw: the attack he witnessed in the gallery was not Monica being assaulted but rather Monica attacking her husband, who was wearing the raincoat. She flees and he pursues her to her art gallery. There, he is trapped, pinned to the floor by the release of a wall-sized sculpture of wire and metal. Unable to free himself, he becomes the prey of the person he was pursuing. This climax to the mystery, with strong sado-masochistic elements, has the knife-wielding Monica teasing Sam as she prepares to kill him. As she raises her knife, the police burst in and apprehend her, notified by Giulia, who had escaped. Sam is freed and Monica is taken to a psychiatric hospital. The victim of a traumatic attack ten years before, seeing the painting of the murdered girl drove her mad, causing her to identify not with the victim but with the assailant. Alberto likewise suffered from an induced psychosis, helping her to cover up the murders and committing some himself. Sam and Giulia are re-united and return to America.

Question: Who does Sam receive menacing phone calls from?
Answer: The killer