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O2 customers in Birmingham have been listening in to callers in Scotland with the kind of crossed lines not usually experienced on a telephone network for decades.

The problem isn't widespread, but O2 has confirmed that customers around Birmingham have found themselves lurking on Scottish calls in an unrequested party line, able to listen in to what should be private communications without the knowledge of those making them.

Crossed lines get their name from the first automatic phone networks, based around a "crossbar" switch. Connections were established using a frame with two layers of parallel wires, giving the appearance of a grid when viewed from above but actually just apart from each other. A hammer at each cross point could push the wires together, making an electrical connection, and banks of such frames connected phone calls.

But two hammers pushing two intersections with only one intersection between them could create a contact at that point, a "crossed line", connecting two separate calls into a single party line.

It seems very unlikely that O2 has been running its network over networking kit approaching its centenary, entertaining as that thought is, so the problem here is obviously more digital in nature but it seems a good deal harder to track down.

Reports of similar problems on O2 go back to 2010 at least, pointing to some long-term but very rare problem that flares up every now and then. O2 say it'll update us when it can, but in the meantime anyone discussing assassination plans or dead-drop locations should stay off O2 especially if they're in Scotland. ®

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