It's been fairly well established that numbers stations (or at least most of them) use OTPs. The Soviets actually pioneered their use in sending sensitive information to both agents and 'dipolomatic' recipients. No doubt the many Russian operated numbers stations, such as the English Lady still use them today. The Russians really know how to run Spy Numbers Stations. Sadly, they've failed to impart much knowledge to the Cubans, their Atencion Stations are more likely run by the Three Stooges.

The general theory is that a so called "dictionary" is used to convert words or phrases to code numbers. Unlisted words or names are spelled out. These codes are then "modulated" if you will by a series of random numbers - the One Time Pad. It could be as simple an operation as adding each dictionary output number to the corresponding number in the OTP (ignoring carry). I briefly touch on this. The reverse process is used by the recipient.

It's quite likely that the bulk of the transmissions observed are dummy traffic. This serves several purposes - first, it makes traffic analysis more difficult. Otherwise, when something important was going on, the "other side" would notice a sharp increase in transmissions. Interesting, this effect IS noticed with the MOSSAD stations from Israel. The so-called 'Message String' transmissions appear whenever there are increased tensions in the Middle East. Second, it can give the appearance to the other side that you have many more agents than you really do.

"Between Silk and Cyanide" by Leo Marks is a must read. He worked for British Intelligence during WWII, and ran the department that communicated with British agents in Nazi occupied Europe via radio. At that time, they did not use OTPs, but instead used a convoluted method involving poems as the encryption/decryption key. But Marks does touch upon OTPs near the end, it seems the British finally moved towards them. The present Lincolnshire Poacher and Cherry Ripe stations can probably trace their origin back to Mark's operations.

Any transmissions encrypted using a OTP are not breakable. Period. (Yes, you could try to bribe someone for the message, but that really isn't breaking it) It's the most secure encryption system possible. Yes, it's cumbersome, having to deal with shuffling around the pads, but if handled properly, 100% secure. No other encryption system can claim that.