Busy signal

The busy signal has become less common in the past few decades due to the prevalence of call waiting and voicemail.

[1][2] Due to a flaw in the telephone switching equipment, teenagers discovered they could talk to each other over the busy signal, often exchanging phone numbers, mostly for the purpose of dating.

Common phone numbers for this to form were on popular radio station request lines, where teens would be calling in en masse to try to win concert tickets or request their favorite songs, thus "jamming the lines" and generating a perpetual busy signal.

This sequence was already in use in Portugal, Spain, The Netherlands, (West-)Germany, Italy, Greece, Switzerland, Lichtenstein, Iceland, Norway, Andorra, Monaco, San Marino, and Vatican City prior to publication of the ETSI recommendation.

Historical oddities within the EU are: The ETSI recommendation is also the default (i.e. non-localized) busy tone generated by mobile phones that follow the GSM & 3GPP family of standards.