I would definitely suggest having a staffing or on-call plan in advance. Trying to fill holes by the seat of your pants is never a good plan
I've only worked on one closed unit. It was a low-risk LDRP with a level II NICU. There had recently been an influx of new staff members, most of whom were not yet fully oriented to every aspect of care (ante-partum, L&D, mother-baby, NICU, C-section, and PACU) so it was imperative to have a call schedule that took each nurse's skill set into consideration. We worked 8-hour shifts, and took 8 hours of call per week, in 4 hour blocks. Most people took their call immediately before or after (or both) a regularly scheduled shift, creating the possibility of a 12 or 16 hour shift once a week. The hospital provided pagers, and there were a sufficient number of them that we didn't have to make a special trip into work to return them if we weren't called in. We just brought them back on our next shift. On call was not used as the method of choice for filling a call-out. The charge nurse tried to find a substitute, and called the on-call person only if there had been a sudden increase in census.
As far as floating to another unit, we could if we wanted to. A few of our nurses had previously worked med-surg in that hospital and didn't mind doing so. Most of us preferred to stay home rather than float if our census was low.