They record all foreign diplomats. This is how he got caught. They weren't recording Flynn. They were recording the diplomat he contacted
Its in the Washington post report itself lol.
Although it remains unclear whether Flynn himself was being monitored for any reason, his conversations with Russian Ambassador Sergey Kislyak were intercepted because the Russians’ calls are routinely monitored.
Do you just post stuff without reading your own links?
If you're calling known Russia agents or Russian government officials, your call is being monitored.