I Was a Teenage Werewolf

Tony Rivers (Michael Landon), a troubled teenager at Rockdale High, is known for losing his temper and overreacting. A campus fight between Tony and classmate Jimmy (Tony Marshall) gets the attention of the local police, Det. Donovan (Barney Phillips) in particular. Donovan breaks up the fight and advises Tony to talk with a "psychologist" that works at the local aircraft plant, Dr. Alfred Brandon (Whit Bissell), a practitioner of hypnotherapy.
Tony declines, but his girlfriend Arlene (Yvonne Lime), as well as his widowed father (Malcolm Atterbury), show concern about his violent behavior. Later, at a Halloween party at the "haunted house", an old house at which several of the teenagers hang out, Tony attacks his friend Vic (Kenny Miller) after being surprised from behind. After seeing the shocked expressions on his friends's faces, he realizes he needs help and goes to see Dr. Brandon.
On Tony's first visit, however, Brandon makes it clear that he has his own agenda while the teenager lies on the psychiatrist's couch: Tony will be an excellent subject for his experiments with a scopolamine serum he's developed that regresses personalities to their primitive instincts. Brandon believes that the only future mankind has is to "hurl him back to his primitive state." Although Brandon's assistant, Dr. Hugo Wagner (Joseph Mell), protests that the experiment might kill Tony, Brandon continues and within two sessions suggests to Tony that he was once a werewolf.
That night, after a small party at the haunted house, Tony drives Arlene home; and one of their buddies, Frank (Michael Rougas), is attacked and killed as he is walking home through the woods. While Donovan and Police Chief Baker (Robert Griffin) review photographs of the victim and await an autopsy, Pepi (Vladimir Sokoloff), the police station's janitor, persuades officer Chris Stanley (Guy Williams) to let him see the photos. Pepi, a native of the Carpathian Mountains, where werewolves, "human beings possessed by wolves", are common, immediately recognizes the marks on Frank's body, much to the disbelief of Chris, who balks at the idea of a werewolf.
The next day, after another session with Brandon, during which Tony tells the doctor that he feels that there is something very wrong with him, Tony reports to Miss Ferguson (Louise Lewis), the principal of Rockdale High. Miss Ferguson tells Tony that she is pleased with him; Brandon has given him a positive report regarding his behavior; and that she intends to recommend Tony for entry into State College. As Tony leaves the principal's office happy with the good news, he passes the gymnasium where Theresa (Dawn Richard) is practicing by herself. A school bell behind his head suddenly rings, triggering his transformation into a werewolf, and he attacks and kills Theresa. Tony flees the high school and, despite the changes in his facial appearance, witnesses identify him by his clothing, causing Baker to issue an all-points bulletin for his arrest.
A local reporter, Doyle (Eddie Marr), interviews Tony's father, as well as Arlene and her parents, in the hope of locating Tony and getting a scoop. Baker and Donovan attempt to trap Tony in the woods where they think he may be hiding. Still in the form of a werewolf, Tony watches as the dragnet looks for him, but is surprised by a dog and ends up killing it.
In the morning, Tony awakens and sees he has reverted to his normal appearance and walks into the town. After phoning Arlene (who answers, but refuses to tell the police who is on the line), Tony heads to Brandon's office and begs for his help. Brandon wants to witness Tony's transformation, and capture it on film in order to advance himself in the scientific community. Brandon tells Tony he will help him and after telling him to lie on the couch, injects him with the serum again. Immediately following the transformation, a nearby ringing telephone triggers Tony's instincts and he leaps up---and kills both Brandon and Wagner---breaking open the film camera in the process, ruining the film. Alerted that Tony has been seen nearby, Donovan and Chris break in and are forced to shoot several times as Tony advances toward them. Upon dying, Tony's normal features return, leaving Donovan to speculate on Brandon's involvement â and on the mistake of man interfering in the realms of God.

Question: What triggers Tony's instincts when he's on the couch?
Answer: A ringing telephone