I haven't read into the extent of cell phone monitoring, but the crazy part of this corporate datamining industry is that they are open with you if you can comprehend (or find) the privacy policies. for modern smartphones, the developers of grapheneOS are quite aware of what they do with a radio being on the board so that network controls are ineffective in stopping tracking if they really want to monitor you. Mind reading is likely not possible yet or if it is it's not widespread but the burden of proof doesn't really come into play here because both the companies themselves and people who work on mitigation of that tracking. It is more likely cookies and things like that are telling on you than actual voice listening.
That being said, most people who are interested in privacy are doing it for
>>118116 -like reasons, it's more about avoiding corporate datamining for marketing than a government or LE, which I imagine are the only ones who would use your cellular network to target you.
Like in vid-related's point about email, if you are doing something that the state or other powerful adversaries should not see, do not do it on a mobile phone, the average privacy conscious user probably isn't on the radar of even LLE.