It's important to agree on this one. Maybe you've got a case of mild diarrhea, nothing severe, and no vomiting. You aren't sure if you should take medicine or just bear with it. Many doctors allow their nurses to offer phone advice, and this saves you a trip, and prevents the doctor from seeing cases which could be treated at home. If you want to speak personally to your doctor, it is nearly impossible to do so during the day.
Many doctors call their patients back at night or the next day.
That is acceptable, but it's important to find out what your doctor feels is acceptable. There is a reason that doctors' waiting rooms are stocked with reading material. Many patients spend a lot of time waiting to be seen for a very brief appointment. Ask your doctor what he or she believes is a reasonable waiting time. No one likes to plan a hospital visit, but sometimes they are necessary.