According to the BBC the 7/7 investigation involved the collection of a very large number of exhibits, including taking over 18,000 statements.
The BBC stated that over 90,000 phone calls were examined and 4,700 phone numbers “probed”.
90,000 phone calls, assuming there are two people on each call and not conference calls with multiple people, means that the police processed the phone records relating to upto 180,000 people.
While tracking calls and similar information is critical in an investigation, 180,000 people is a fairly wide net to spread. With a UK Muslim population of just 1.6 million, the phone records represent over 10% of the population.
With story after story of mistaken identities and wrongful arrest, when the net is case very wide there is the inevitable result of mistakes being made. The issue was also addressed in a 2003 report into the terrorism legislation in the UK.