Registered Nurses always full-time?

Maybe this is a dumb question, because I am only just thinking about going to school for a career in nursing (specifically something in women's health, like a nurse-midwife), but is an RN generally a full-time, on-call type of position? Or is it fairly common to work part-time (20-35 hours a week) and not be on any on-call type of status?
There is alot of part-time available in nursing, its usually cheaper for hospitals to hire part-time, there is usually little to no benefits costs that way. I am not too sure about the nurse-midwife role, (b/c in Canada we unfortunatly don't have nurse-midwives. We have midwives but it's separate from nursing.) Also, when you are part-time they may call you to come in, but usually its up to you to accept the shift, you are not obligated unless you are getting paid to be "oncall" (such as in the OR). Most full-time workers are not called in to cover shifts unless they are desparte to fill a shift, because it usually cost the hospital overtime pay. I hope this helps.