@leymoo Capital phone-ins are mentioned (with photos) here:http://www.samhallas.co.uk/repository/journals/Post_Office_Telecommunications_Journal/POTJ%20Vol%2025%20No%204%20Winter%201973-74.pdfThe unit with all the switches in the bottom left of the cover photo is the key-and-lamp unitThe use of KLUs changes my technical theory, but there are other options. the announcer mentioned in the article may have been switched off or faulty, or multiple keys on the KLU could have been left operated, or something else entirely!This has been quite a fun little technical research jaunt!