For some reason this question gets asked a lot here; I would surmise that you are referring to encryption when you say "coded". I think the FAQ covers this topic, while there are plenty of online discussions of the 'how' of encryption methods. Some agencies over-use encryption while others wisely save it only for special situations (major terrorist incident) or units (surveillance, SWAT) Hands down, it is illegal for a listener to decrypt any encrypted transmission, unless you happen to be a government agency with the authority to do so. There are some voice de-inversion units which would at least be gray tech, if not entirely illegal, that can undo old voice inversion privacy methods, but everything else is essentially out of reach.