I don't know if that lead was explored, the article sidebar hints at it:

[...]
Another popular theory posits that the U.S. National Security Agency, Central Intelligence Agency, or some other U.S. spy agency did it. The location of the monitored phones correlates nicely with apartments and other property under the control of the U.S. Embassy in Athens.
[...]

If the shadow phones number in the hundred some things becomes really hard to hide. And conspiracy theory of having the real spy hiding all the shadow phones near USA property undiscovered for more than a year harder to believe.