In the world of surveillance, there are rarely any absolutes — and that applies to guarantees, too. If NSA, GCHQ and associated agencies really want to get at your communications, and are prepared to invest their resources in doing so, they will usually succeed to some extent. But there is no need to be fatalistic about it: some common-sense steps will go a long way toward frustrating most kinds of electronic surveillance.
Above all, it is important to bear in mind that even in these days of highly automated surveillance, the final step at least has to be undertaken by humans. They remain a scarce resource. So, while it is true nowadays that your phone calls could quite easily be automatically transcribed with a fair degree of accuracy, the quality of that automatic transcript will take a nose dive if you vary the acoustic environment unpredictably.
For example, you could speak with some music playing in the background, or running the words into each other, or by hamming up a strong accent that does not have many millions of speakers and that therefore will trigger a less accurate response by voice transcription algorithms. If you speak obscure foreign languages, sprinkle expressions and sentences in those languages into your speech. Basically, imagine the crystal-cut delivery of news announcers in days of yore — their pitch, their speed, their intonation — and do the exact opposite of them.
You could also revert to old-fashioned techniques for obscuring words, such as playground methods of putting extra syllables into words (e.g. aygo-paygo language). Do not do this just when you have some gobbet of information to hide; instead, do it randomly and with a lot of furtiveness, while what you’re actually saying is junk or humorous information.
Another way of frustrating the humans at the end of the technological chain of voice retrieval is by gassing endlessly about irrelevant or insignificant things in your call — particularly at the beginning and end. That way, you can stuff your sensitive information carefully into the middle sections of long calls with more confidence than if you were just to call your contact quickly, drop the information purposefully and hang up. Needles are best hidden in haystacks, after all.
The choppier your communication patterns are, the harder they are for computers or human analysts to detect rhyme or reason in. Call a contact up and refer darkly to “the message I sent you last week on secure channels”. Make sure he is in on it, so that he plays along by muttering a conspiratorial acknowledgement. Any officers assigned to you will then have the chore of combing back through other data repositories in a wild goose chase for your non-existent message. Even better, you could send such a message with some DIY encipherment, only for that message to turn out to be gobbledegook. Such a profile will help to have you written off as a time-waster.
Now, assuming that an intelligence officer really is listening to your calls, after having seen that the automated transcript is garbage because of the countermeasures you’ve taken as above: how to bore the pants off him? The best ways to do this are to witter on at length about obscure hobbies or religious views. This might cause him to lose the will to live, or at least the will to listen to you again tomorrow and the day after that. You could even address him directly, offering an array of eyebrow-raising conspiracy theories to brighten his afternoon.
You can take this a step further. In a panicked voice, you could predict that something awful and perhaps illegal is going to happen on a certain day soon, which you could describe in lurid detail. Entirely made up, of course. Cometh the hour, happeneth nothing at all. You have just trashed your track record — and have put another nail in the coffin of intelligence agencies’ preparedness to follow your every word.
Of course, while these techniques might reduce the likelihood of intelligence officers wanting to stay on your case, you should also take serious steps to secure your communications where key information is involved. For that, encryption and the use of a wide range of communication methods — including written notes — is the winning combination.