Never leave without an appointment. When doing routine things like the dentist or yearly car inspection make the next appointment on your way out. If booking your next dentist visit 6 months out you get your choice of any time you like. Just stick it in your calendar and move on.
The only time that voice doesn't work is if the people you're making an appointment with only schedule out a certain time in advance and you need to go out longer. The cardiologist office I go to only ever lets you schedule 6 months in advance and I gotta go yearly, so I don't have that luxury.