Night Caller(1976)

Robert is a lonely young man who takes pleasure in making obscene phone calls to strangers. His obsession, however, is the beautiful woman who lives across from him. After spying on her making love to her husband, Robert arranges for the two of them to accidentally meet. Beginning a slow and calculated seduction effort, Roberts grip on fantasy vs reality begins to slip, sending him into a downward spiral of increasingly perverse sexual desires.