yep the “fingers in ears” sign (one of my favourite “stick men in peril” ones :laugh_at:) is supposed to be affixed to the leads of pairs of cans or near the headphone outputs of mixers etc in BBC facilities, although some other broadcasters use it these days.

They do have to be a bit more careful at Auntie, its not uncommon for engineers to send very loud tones down audio circuits to test them and also to ensure that British Telecom don’t clear down (hang up) / reprovision (allocate to someone else) a distant audio circuit which may be travelling through one or more Telephone Exchanges along with everyone elses phone calls and broadband.

these days such a circuit (usually ISDN2) is often seized and left open some time before the remote broadcast commences… it appears to the exchange as a normal “phone call” but with many digital exchanges if you leave a telephone “off the hook” during a call and there is just silence, BT will clear the circuit after about 10-30 minutes to save peoples phone bills and release their equipment for more productive traffic (it might be needed for a 999 or 112 call!)

one common form of this signal is a pip at 1Khz, 0dB every second which most people are familiar with 😉

also a digital audio circuit where the clock has slipped will produce random noise pegged at the highest audio level. If this is fed to analogue convertors and then to the headphones or a telephone handset it will give the listener a nasty shock (actually called an “acoustic shock…”)