... Sometimes get one and there's no-one on the end of the phone.
This can easily happen with 'legit' callers, especially where it is a company (or call centre acting on behalf of a company) that uses an auto dialler. These thingies educate themselves into how long to the second each call should take, continually revising their prediction, and ditto for how long from start of dialling the next call to answer. Suppose it takes eight seconds to actually dial and for the call to connect and five seconds ring tone before the average punter answers. The system will therefore put the next call in thirteen seconds before it calculates the next operator will finish their current call so the machine can immediately pop the target's details up on the operator screen in that magical instant when the call is answered and the operator becomes available.
Which goes to show how stupid supposedly sensibe people can be. Calls may be longer or shorter but very rarely exactly on the average, and time to answer is not really that accurately predicatable. So it may be several seconds (or many seconds) after a call is answered before there is an operator to start talking to you. Or (averages are a beggar, aint they) some calls are finished sooner than expected and the operator is ready before a call is. This is a bad thing of course for them wot run the call centre as it represents dead time in which the call centre staff are being paid but not actually in a call.
Annoying, but not always sinister, just inadequate technology, unrealistic call centre managements and attempts to model tightly things that are inherently too variable.
Waffle over. Sorry but these ruddy things are a pet hate of mine! (you'd never guess)