Located inside the metro network of Madrid, in the "Callao" station near the exit to "Gran Vía".
In Spain, is almost impossible to call to the payphones on the street (if you try it, you'll hear SIT tones). However, I found that about the 50% of the payphones inside the metro are callable. This one not only is callable but also it RING LOUD and the audio works OK both ways. The "Callao" station is a busy and central one (many tourist and locals). I guess that from 6:30 am to 1:45 am (almost all day) there are good chances that someone will respond.
Many thanks Santiago for this great new addition. We hope and look
forward to many more from you.