Original: 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.
Simplified: 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.