Therefore, I am confused whether or not a warrant is needed. If the phone networks were straight up selling call records, then surely no law enforcement agency would bother with warrants.
Call records have never needed a warrant (lookup "pen registers"). Call contents (i.e. wiretaps) have in principle always needed a warrant, modulo many exceptions. These days though, call records seem to be for sale to anyone who wants them, whether or not that is legal.
A private investigator could fill us in, but if you look at the different personal data services you'll see a lot of "check this box to agree that you have a valid legal reason to pay us $75 for your ex's info" type setups. Pirate Code law.
https://archive.nytimes.com/www.nytimes.com/interactive/2013...
Therefore, I am confused whether or not a warrant is needed. If the phone networks were straight up selling call records, then surely no law enforcement agency would bother with warrants.