When I was at my old hospital (large teaching hospital, about 20 techs all shifts in BB), unexpected callouts were covered by the tech on call (if it left evenings with <5 or nights with <2, otherwise they just dealt with it). One day shift tech took call for a week at a time. Planned vacations and general short-staffing on eve and night shifts were covered by volunteers first, then a list. How the list worked was if you covered an extra day, covered a different shift, took 7 days of call, etc you got points and moved down the list. If a day needed coverage and no one wanted to do it, then the person at the top of the list would have to cover. Fortunately we rarely used it because we had so many techs who were willing to help out. The system of on-call and the list worked out really well for us.