Both organisations also provide 'reserve' or 'support' cover to some, though not all, of the ambulance trusts (dependent on the local MOU), where ambulance crews from one of the organisations (who are usually volunteers, but in some instances may be paid staff) will attend 999, GP Urgent or PTS calls on behalf of the ambulance trust. In these cases the organisation is paid by the trust. This service is most often called on during major incidents, when there is a high level of staff absence or when there is an unusually high call volume, although in some areas, voluntary crews are regularly used to supplement full-time trust cover.