From the time the incident is confirmed as a false alarm then the role call may cease. It is a legal duty to have measures in place to ensure that the evacuation is complete and no persons are at risk. As Martin says this may be a nominal roll call or it may be by wardens sweeping the building. Neither system is perfect. Wardens cannot safely sweep an area where there is a fire raging- and this is precisely where persons may be trapped. So a sweep search on its own may not be enough. On the other hand a roll call is only as good as staff discipline and booking in / out procedures, so a combination of both is best.
Up until the incident is confirmed as a false alarm then procedures should be followed. If you then record the time of alarm, time taken to account for staff, persons taking part, problems encountered and lessons learned you have carried out your evacuation exercise for those staff. Another tick in the box.