ABC News mentions Echelon

I just heard this from a commentator on ABC News:

[US Intelligence] has the ability to "Sweep up all communication going
around the world in a very broad way."

There are currently "millions and millions of phone calls in this vacuum

"They will be sorting through all of the communications going through phone
numbers that they have an interest in."

The reporter also mentioned that the officials have to be careful not to
intercept calls of US Citizens.

I'd love to know the technical issues on how you determine, given only a voice to go on "is the caller or the recipient a US citizen or a foreign national?"