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Old 19th May 2019, 10:41 am   #19
Join Date: Jul 2007
Location: Flintshire, UK.
Posts: 560
Default Re: GPO/BT Call Progress Tones?

Originally Posted by G6Tanuki View Post
In past-times the UK phone-network would include very short 'pips' of in-band audible tones for various signalling purposes during call-setup and cleardown.

Though these have not been used for a few decades, it's still quite common for the sound-effects-people in radio-dramas to include them in "calling someone on the phone" scenes.
Those tones were initially AC1 signalling system tones and in later years, other AC signalling systems - AC3, AC9, AC13 etc - all for passing the tones over amplified /carrier/co-axial circuits that had no DC path for signalling backwards and forwards. In theory, callers were not supposed to hear them as they were not part of the 'call progress' tone system.
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