Just try to figure out how difficult it would be to actually capture 100% of a country’s inside and outside telephone traffic. Think about North Korea, who were actually technically still at war with. It might be a good candidate for the country in question until you figure out that they share a border with China, are insular and famously inscrutable. Tapping all the telephone lines between those two countries isn’t something I’d ever be anything near 100% certain of.

The only real remaining candidates are Afghanistan where we still operate militarily and could tap the official civilian infrastructure, and strong allies like the UK and Australia who might conceivably just give us such access freely as part of their own counter terrorism operations.