One day Earth is visited by a >4th dimensional alien entities, they do not have a physical body and all we know is that they can cause the air around a person to vibrate at different pitches seeming imitates voices, not to be mistaken for telepathy.
They are mimicking your own voice when communicating with you and you only, each individual is capable of talking to multiple persons any where in the world simultaneously (latency applies due to speed of causality).
Only a fraction of them (alien entities) managed to control their telekinetic power so that the air won't fatally scald a person they are talking to, think of Superman hypnotizing himself with "mental block" as he believes he is living in a world of cardboard.
Because of the great difficulty in controlling their ability they usually reuse sentences that you have said and limits communication to within 30 minutes fortnightly.
How can any human knows that it is indeed the encounter of the "<3rd" kind and not some mental illness or "mass hallucination?