How did Bellingcat obtain telephone metadata of FSB operatives in their investigation of Alexei Navalny's poisoning?

[cirosantilli]

https://www.bellingcat.com/news/uk-and-europe/2020/12/14/fsb-team-of-chemical-weapon-experts-implicated-in-alexey-navalny-novichok-poisoning/

https://www.quora.com/unanswered/How-did-Bellingcat-obtain-telephone-metadata-of-FSB-operatives-in-its-investigation-of-Alexei-Navalnys-poisoning'

https://twitter.com/bellingcat/status/1144697596109295618 says on a previous investigation that the "Bellingcat obtained telephone metadata records for Sergeev from a whistleblower working at a Russian mobile operator". So they got that tip again? That seems really odd. The FSB can't be that stupid?

https://www.quora.com/How-did-Bellingcat-obtain-telephone-metadata-of-FSB-operatives-in-its-investigation-of-Alexei-Navalnys-poisoning/answer/Centis-Gerc%C4%81ns puts forward the suggestion that they bought cell phone data only for the involved agents from corrupt/whilstleblower employees/officials