Across many frequencies, from many locations, and with no apparent purpose, numbers stations broadcast numbers, characters, and occasionally specific sequences of random noise. It is speculated they are used by secret agencies to communicate with their agents, by criminal cartels for essentially the same purpose, by AIs as part of some ancient infowar scheme, or by anarchist pranksters to waste PC resources by making them investigate meaningless signals. Each numbers station is unique, and they broadcast from within many different polities, so all possibilities could be true
Most likely, these broadcasts are made for use with one-time pad systems. These days the system is automated like anything else: usually either one's muse stores the key, and automatically translates messages for you. The broadcasts are still meaningless to anyone without the key, but infosec needs to become a priority, to ensure no one can hack your muse.
Plot Hook: Whether it is their purpose or not, numbers broadcasts are tying up Firewall predictive resources on random noise. Firewall needs to shut down as many of them as possible, despite the fact that they many are operated by the more powerful organizations of the system. Firewall needs a team willing and able to undertake a series of raids against high-security numbers stations across the system.
Plot Hook: Firewall operates its own stations, for use with one-time pads. Recently, they have been used as part of an operation tracking down a powerful asynch. Impossibly, the asynch seems to have discerned the meaning behind the broadcasts and killed the Firewall agent tracking him. The mission to bring in the agent has become a priority, as Firewall needs to learn how he was able to crack the code.
Plot Hook: One broadcast is maintained by the ID Crew, an automated voice sending a rapid stream of digits from 1 to 8. It is speculated that they use the broadcast to transmit data and/or computer code, which would be slow but, assuming they are using a one-time pad system, very secure. Anyone who manages to steal the key and decrypt their data would gain a great deal of rep.
Notes: Inspired by the real numbers stations.