Very much can be done on a case-by-case basis: Get proper warrant, ask telecom provider to provide call logs for customer under investigation. Analyse. Ask for

Very much can be done on a case-by-case basis: Get proper warrant, ask telecom provider to provide call logs for customer under investigation. Analyse. Ask for call records for other customers that seems to be interesting based on initial study (if necessary after getting extended warrant.) None of this needs a constant omnipresent dragnet ir secret courts interpreting secret laws in secret ways. I'd go so far as to say it is dangerous to spread the idea that this is necessary.