Well, the WP article on NSLs has an EFF-sourced minimally blacked-out example of an NSL, which explicitly states "[blacked out communication details] or through

Well, the WP article on NSLs has an EFF-sourced minimally blacked-out example of an NSL, which explicitly states "[blacked out communication details] or through use of a delivery service or secure fax...", and while there is an entreaty to not disclose it through routine mail or phone, it would seem there are mechanisms (including, presumably, asking your attorney to file paperwork about it and seeing if it gets sealed so fast your head spins) to communicate about it.