Speaking from a telco perspective, all of our switches make a CDR (call data record) for ALL calls in and out. Origin number, destination number, number of seconds, date stamp, etc. etc.
There are millions of CDRs per day.
It would be trivial to filter them for calls to/from certain ranges of numbers.
EDIT: To be clear, it makes no difference if your call is one second or one hour, I can easily find the CDR for it and prove you made/received a call from/to X
You are correct, of course, except that I highly doubt North Korea has access to Chinese telephone records. As another commenter pointed out, the North Koreans are probably relying on radio triangulation. With the right resources, I'm sure they could detect that a call is occurring almost immediately, and perhaps even pinpoint where it is coming from, but they still have to go out and catch them. I don't know what resources the North Koreans actually have, but I suspect they are fairly limited.
There are millions of CDRs per day.
It would be trivial to filter them for calls to/from certain ranges of numbers.
EDIT: To be clear, it makes no difference if your call is one second or one hour, I can easily find the CDR for it and prove you made/received a call from/to X