As Mark Steyn pointed out in a recent interview, a domestic phone number is not assurance that the person who answers is a U.S. citizen...
I think you're missing the distinction between "domestic" and "foreign", as they pertain to the present matter involving intelligence gathering.
The difference between "domestic" and "foreign" national security intelligence intercepts has nothing to do with the point of origin, destination or interception of the communication. Nor does it depend on whether either party is a U.S. citizen or resident. The difference between "foreign" and "domestic" national security intercepts, as the term is used by the courts and intelligence agencies, is the source of the threat, i.e., whether at least one party to the communication was acting as, or on the behave of, an agent of a foreign power.
That distinction was never made more clear than it was in the Truong case. Truong, a U.S. resident alien, and Humphrey, a U.S. citizen and an employee of the USIA, conspired to commit espionage by delivering confidential government documents to the communist government of Vietnam from 1976 to 1977. In this case, the court held that the intercepts did not require a warrant since both defendants were acting as agents of a foreign power and thus the intercepts were a legitimate exercise of foreign national security intelligence gathering.
Note that in Truong, both the origin and destination of the intercepted calls were within the United States. Note also, that both defendants were U.S. residents, and in Humphrey's case, a U.S. citizen, as well. Yet the court held that these were foreign national security intercepts.
(p23185: I pinged you only because of your expressed interest in the matter.)