Answer:
variable-interval
Step-by-step explanation:
Four types of reinforcement schedules exists:
1. Fixed-ratio schedule.
2. Fixed-interval schedule.
3. Variable-ratio schedule.
4. Variable-interval schedule.
Variable-interval schedule: This is one of the reinforcement schedule which is used in operant conditioning. In a variable-interval schedule, the reinforcement is being given to a response after every particular time that has been passed, and the time provided is considered to be as unpredictable, yet the time is classified to be as on a variable or changing schedule.
Hence, in the given question, holding the phone line is reinforced on a variable-interval schedule.