A numbers radio station is a signal that transmit some type of code. There are quite a few of these and some of them have been operating for quite some time. UVB-76 is one that transmits from Russia. It has been playing a buzzer sound since the late 90’s and occasionally there are voice transmissions.
Earlier this year it stopped transmitting, but now it seems be back and people are hearing a lot of background noise–almost like there is a microphone left on in a storage room somewhere that everyone has forgotten about. You can listen for yourself here. An archived Russian web page seems to indicate that the station was used to send out military orders and the time they were to be executed.