Imagine making a call to a friend in downtown Pyongyang from just across the border in Seoul. That reality might be a way off yet, but if things did mellow between the two nations, at least they’d both be up to speed when it comes to the latest phone technology to aid their communication. Not so long ago, even having a landline phone at home in North Korea was considered a luxury, but in recent years a mobile phone boom has taken off at a pace that is as speedy as it is surprising.

Anyone trying to make sense of the rapid spread