When cell phones first became big, probably 10-15 years ago at least, there was a website for my area I lived in at the time (southern Illinois) that would list texts and people could vote on the funniest ones. There were some really private messages that would hit the top (obviously phone numbers weren’t displayed.) So it used to be people had the assumption that texts were public, because for some carriers they basically were.
If I understand correctly, the initial telephone systems were run by manual operators at a physical switchboard, who could listen in to anything that was said on any line. Many people also had party lines, where someone (in another house or apartment) could pick up their phone and listen to your conversations.
So, no, not much of an expectation of privacy - at least, there shouldn't have been.
I'm under the assumption that wiretapping to create evidence is illegal, but wiretapping to get a warrant probably happens all the time. (AKA Judge and Police officer listen to illegally captured audio - Judge approves official warrant to make future recordings legal)
Not sure which is why I'm asking.