Both organizations 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 organizations ( 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 organization is paid by the trust . Both organizations 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 organizations ( 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 , with the organization receiving recompense from the trust .