Pedro often has a hard time with the V2a, M8a, and SK01 transmitters, this is why the audio fairly often has some kind of problem. It is not unusual to hear audio crosstalk form other, presumed, co-located stations such as Radio Habana.
The "phone ringing" has been noted for a while now. No one I have heard from knows forr sure what is up with it and it has been seen on multiple frequencies. Not just the Spanish numbers freqs but also several Radio Habana frequencies.
I myself am not sure there is not some kind of data on the "phone ring". I watched it closely for a while one day and noted different numbers of pulses and even steps inside each ring pulse. Also, some ring pulses were longer or shorter than the average.
I am not saying the phone ring pulses are intended for the same recipients as the numbers transmissions. What I am thinking might be happening is that a data line coming to the studio is cross talking into an audio line. It is just a possibility.