Experiment in Terror (1962) FFF

San Francisco bank teller Kelly Sherwood returns home one evening to be assaulted, her attacker who, in his threats, knows all about her - her movements and that of her teenage sister, Toby. Those threats include killing her and/or Toby if she contacts the authorities or doesn't carry out his plan for her, the details for which he will relay in subsequent contacts: rob her bank of $100,000, he "kindly" cutting her in for twenty percent. Through this ordeal including subsequent contact, she is never able to see his face, but she would recognize his distinctive voice, with its labored breathing, like he is asthmatic. In she telephoning the FBI, Kelly discovers that her attacker is indeed monitoring what she is doing in cutting off the call before she is able to provide them with any information, this her only one strike allowed. Regardless, FBI Agent John Ripley is able to follow up to discover what is going on with Kelly seemingly without her attacker knowing. Thus begins a cat and mouse game between Ripley and Kelly's attacker, any mistake by Ripley or his team which would put Kelly and/or Toby's life in mortal danger.