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 . This service is most often called on during major incidents ( e.g. the 7 July 2005 London bombings ) , 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 .