OK, When you sign aboard for any military spot, everyone signs for 8 years the first contract. Yes, eight years. However, how that eight years is done is where the details lie. Most do four years active something (active duty or active reserve duty) and the last four years as innactive reserve. In general, the innactive reservist does nothing more then complete a medical physical exam every year or so....depending on your skill.
If you look at who has been actually called up for duty out of the reserves the most, guess who....physicians and other healthcare specialist because the military can not pay enough to keep these highly skilled people beyond their commitment. Thus, innactive reserve does not mean you will not be called. There are some fields of medicine in which they only have one physician covering several states for the field. Thus, you can be sure they would call up an innactive reservist.