How did you come to that conclusion?
Unless there’s evidence to the contrary, nowadays I would assume almost all telecommunications providers log the metadata of all phone calls.
Anyone can download an app that records the contents of phone calls, and then hit the record button. This may or may not be legal, but capability certainly exists and enough people do it.
Banks, call centers, government agencies, prisons etc. routinely collect people’s voice prints or record the contents of phone calls. Some of them turn off recording upon request, while others either claim they are compelled to record the phone call, outright refuse or pretend to comply.
In the US, the government spies on phone calls involving foreigners, and the NSA is reported to have voice recognition capabilities and can search back in time.
Based on the above, I would say phone calls are normally not private enough, with some niche circumstances where they are private enough.
As others have suggested here, pinky-swear promises that telecommunications providers, intelligence agencies, other institutions and anyone who intercepts phone calls won’t monitor phone calls are not good enough. Cryptography is a requirement for privacy of phone call content. Unfortunately not much can be done about metadata logging and recording by the other parties to the phone call.