I've been a volunteer myself. There were times when I was out of town... Or maybe had a beer with friends... Or I was sick... Or there was some reason that I couldn't respond if the pager had gone off. The same must be true for you. You can't always be available and to suggest that you are isn't fair.
Also, don't paint vollie/career with such a broad brush. In many smaller departments, the off-duty career firefighters are recalled for working fires right alongside the volunteers.