The following is an excerpt from the code of practice in respect of predictive and power dialers, first published by the UK Direct Marketing Association in 1995 and updated in 1997.
If a 'live' operator is unavailable to take the call generated by the dialer the equipment should abandon the call and release the line in not more than one second.
The Dialer must at all times be adjusted to ensure that the rate of calls abandoned is no more than 5%.
Although the UK DMA was the first national marketing association to issue a dialer code of practice, it is now behind the standard set in the US. Specifically it doesn't (yet) prohibit predictive hangups which can be a major source of nuisance calls, but expect this to change.
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