4th Amendment:
"The right of the people to be secure in their persons, houses, papers,
and effects, against unreasonable searches and seizures, shall not be
violated, and no Warrants shall issue, but upon probable cause,
supported by Oath or affirmation, and particularly describing the place
to be searched, and the persons or things to be seized."

Yep, I see it. It says right there that international phone calls to
Al Qaida-connected people shall not be monitored in time of war (unless
a court has taken the time to approve it, of course).