They might have watched The Wire: you page Alice, and she uses a public phone to call you. Undetectable unless you wire all public phones in the city, or someone is dumb enough to always use the same phone (which is what happens in the series; they eventually switch to burner mobiles).
I just assume that ease of interception is tangential to the legal requirement for permission.
Paper mail and landlines are incredibly easy to intercept and tap, but that doesn't make it legal.
> Pretty realistic. Like most of the wire in fact.
The show creator worked for years as a journalist on the crime beat in Baltimore, I expect most of the opsec seen in the series comes from real cases.
> really don't understand why they needed a court order for [wiretapping pagers]
As others said, you need it from a legal perspective rather than a technical one. This is particularly true in the US, where the "fruit of poisonous tree" doctrine is pretty strict: if your evidence was not gathered in the proper manner, it must be discarded and it invalidates any further effort based on it. In specific, wiretapping is illegal even when done by authorities, unless they've been authorized by judges - the relevant US laws were tightened up after it emerged (with Watergate) that president Nixon was eavesdropping on his political rivals.
To be fair, they rotate the burners in the series every 2 weeks and it takes the police more than a week to get up on the new ones.
It was cool to see that it was in fact an opsec fail (the guy buying the phones all over the country got lazy and bought too many from the same shop) to break through that. Pretty realistic. Like most of the wire in fact.
Although one thing in the wire I don't understand. Pagers are really easy to intercept, anyone with a scanner (with discriminator output) can do it and could do it in those times. I did it many times during the days when pagers were still in full swing. I really don't understand why they needed a court order for that (in season 1).