Final answer:
Open-access scheduling is the system that reduces calls to a medical office by allowing same-day appointment bookings, thus decreasing the need for advance calls.
Step-by-step explanation:
The scheduling system available 24/7 that reduces calls to a medical office is a) Open-access scheduling. This system, also known as same-day scheduling, allows patients to make appointments on the day they wish to be seen, reducing the need for patients to call in advance. By offering greater flexibility and immediate access, it often results in fewer calls to the office because patients do not need to plan ahead for appointments. Wave, double-booking, and modified-wave scheduling typically require more advance planning and may not decrease call volume as effectively as open-access scheduling does.