NSA listens to calls inside of compliant countries Australia, NZ, UK, etc and also calls between other countries. I doubt they would have access to calls inside of Syria, but I would not be surprised if Israel had ways of listening to internal calls in Syria. Or Israel could just make it up and tell the US it is so. The US does not seem to be forth coming with their evidence, so I would presume it comes second hand and would not be very convincing in a Court room.
While Israel will almost certainly take no direct part in a military strike, Israeli intelligence information is widely believed to have played a central role in enabling the US’s adamant conviction that Assad’s regime fired chemical weapons at civilians outside Damascus last Wednesday, killing hundreds of people and wounding over a thousand, according to Syrian rebel groups....
