Well, the spies under the direction of England's SOE (special operations executive) spent a lot of their time cutting transmission lines throughout occupied Europe. This was done so the Germans had to broadcast their signals via wireless, allowing Turing and Bletchley Park to intercept and break them much more often.

I highly recommend a book called Between Silk and Cyanide, written by the head of codes (cryptography) of SOE during World War II.