Simplify the following text for a Grade 10 native English-speaking person.
Original: 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 .
Simplified: 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 .