Some dialers dial lots of numbers, allocate connects to waiting agents, then when no more agents are either available or expected to be available imminently, terminate all calls which are ringing and not yet answered.
At some point a call will get clearly terminated anyway if there is no answer, and this issue is all about whether a minimum ring time should be set, obliging a dialer to keep ringing for that length of time to give called parties a fair chance to get to the phone.
Codes of practice are beginning to incorporate minimum ring times, and if hangups occur within these times, then such calls will need to be classified as abandoned. Sytel's view is that the minimum ring time should be at least 15 seconds in all territories, unless there is clear evidence that the vast majority of people answer well within this time.
Hanging up any earlier and having to record an abandoned call is actually an inefficient way of dialing and is usually confined to dialers whose algorithms are unsatisfactory. It's got little to do with true predictive dialing.
But it is surprising just how many dialers hang up within what might be regarded as fair minimum ring times and don't declare them as abandoned calls.