Secret Calls or non disclosed

[martyn94]

I have a variety of responses.
The one that mystifies us is the one that is silent for about 10 seconds, then says "goodbye" in English

I think that these mostly come from call centres which use "predictive dialling". Given that most of the junk calls are very brief, you would waste a lot of time if each caller had to set up a new call once the previous one finished. In order to sweat their staff to the utmost, the call centre has a single computerised system which starts dialling the next mug punter before any agents are free, in the expectation that one of them will be free - on average - by the time the call connects. Quite often, the staff are all still occupied with other calls, hence the mystery cut-off.

Or else they are just ringing round the repertoire of numbers that they bought in some dark corner of the internet to check that the numbers are still "live" before selling the list on to others.