You might be right, but that would be a darn weak protective/preventive measure. No, because a pattern of communication would present the probable cause necessary for a more (rightly) intrusive monitoring program against suspect individuals and organizations using a FISA warrant.
Again I say, you may be right. I was imagining a person avoiding the creation of any pattern, generally by staying quiet. A sleeper, if you will. If it takes one or two coded contacts to initiate pre-planned action, then waiting for a pattern will not result in prevention.
If I am charged with preventing attacks, I want the contents of the communications.
We all envision a terrorist phoning someone in the US on a regular basis. In today's age, I don't think its that simple.
I recall that we were told the terrorists once used pictures to transmit information. Buried in the photo's letter equivalents was messages.
To assume that the terrorists are not expecting us listening in is absurd. They will be taking all sorts of measures to avoid dedection. I would expect multiple partial messages, to multiple recipients using multiple codes. On the surface they will appear to be mundane communications. Combined with other communication, they may have meaning. I also envision that when real messages are sent, an increase of bogus messages will be sent also. It would be impossible to know initially what is what.
Asking FISA permission for any of these monitorings would likely yield a no answer. If it were done once, the administration would have a problem with precedents set.