"But it is interesting to note that every time there is a march, protest or gathering the mobile network operators know exactly how many people attended – even if the police and organisers can never seem to agree."
This is not accurate. Each carrier knows how many of its telephones were connected within the area where the march, protest or gathering took place. This does not account for people who do not bring their telephones, or turn their telephones off. This does not neccesarily account for people who live or work within the area who did not attend the event. This also does not account for people who carry more than one mobile telephone - this is increasingly common as people carry a personal phone and a work phone. This information is good for a basic estimate, but does not directly correlate to the number of people attending a gathering.