Re: GPO/BT Call Progress Tones?
 

I've had several 'exchange equipment busy' tones recently dialled from landlines, either to a mobile or a BT VOIP line.

Re: GPO/BT Call Progress Tones?
 

Lots of the tones are here: https://www.3amsystems.com/World_Tone_Database

Looks, or sounds :-) like North america had one system, most of Europe another, UK a third and other countries copied one of the systems.
Exeption; Germany pre 1979 had a Morse code A as a dial tone. https://bre.is/BUxjDEcuO

Re: GPO/BT Call Progress Tones?
 

I don't think the network actually generates the Pay Tone anymore. Since the 1980s, microprocessor controlled payphones began to replace the old mechanical ones. The new payphones usually required the caller to insert money first (at least the minimum fee), then dial the required number. When the call was answered, the minimum fee was deducted but no pay tone was generated until the time allowed had been used up. This was calculated by the microprocessor in the payphone, either based on the area code + number dialled and the time of day, or by meter pulses sent by the exchange. (Again, I don't think they still have meter pulses.) When the credit was exhausted, a pay tone was generated by the payphone itself, not the network. This prompted the caller to insert more money. If they did not, the payphone disconnected the line after a few seconds, thus ending the call.

There were privately-owned payphones as well as BT-supplied ones, these may have generated a different pay tone to BT. Maybe that's what you heard?

With the older Pay on Answer payphones, the caller dialled the number before inserting money. When the call was answered, the exchange generated the pay tone (400Hz switched on and off rapidly). If the caller inserted a valid coin, the mechanism in the payphone sent a series of pulses down the line, a bit like a rotary dial. Equipment in the exchange detected the pulses from the payphone. If correct, the pay tone was switched off and a speech path was established. At the end of the time allowed for the coin inserted, the pay tone was switched on again by the exchange. If the caller did not insert a coin (or the coin wasn't validated) within a few seconds, the pay tone changed to the Number Unobtainable tone (400Hz continuous) and the call ended.

On the System X digital exchanges, there was a line tester available by dialling 175. (It may still be available, but I don't have an active phone line anymore to check.) After dialling 175 the tester reads out the phone number of the line you are calling from and says "Start Test". Hang up and wait ~ 20 seconds. The tester rings the phone line. On answer, it gives the result of the test, hopefully "OK". Then it says "Dial next test". This is the interesting bit. If you dial 3 (I think) it plays the Pay Tone followed by the message "incorrect coin pulsing". I presume this was for testing the aforementioned pay-on-answer payphones. As I said, I can't try this test now to see if it still works, but if it does, it means there is still potentially a network-generated pay tone. But the strange thing is, I have never, ever come across an old mechanical Pay On Answer payphone connected to a System X exchange. They had all gone by the time System X arrived in my area. Were there any? If not, what was the purpose of the pay tone test? I'm curious.

Re: GPO/BT Call Progress Tones?
 

On my BT landline 175 doesn't do anything, but 17070 confirms the line number and gives a list of options including ring back. I recall from when I worked at a location with a different provider, 17070 just gave the line number message then hung up.