For an appointment reminder service, you might want to check out PhoneTree:
http://www.phonetree.com/healthcare/index.html
It runs off of one of your PCs at the Front Desk and it can give you a report the next day of patients it was not able to reach. This product will also tell you if the phone was busy after x number of tries, or if the phone has been disconnected. This is a good tool to identify “who is missing.”