"anata wa soko ni imasu ka" (Are you there?)
... seems like this "magic spell" works over the radio, which the Festums have to decode and hack into.
I have always wondered if this could be a kind of hypnosis. If so, the Festums are more human like than I think.
The short answer is no, its not hypnosis - its essentially the Festums trying to find/communicate with humans. The long answer is, it has a double meaning in the show.
First of all, the most direct reason that particular phrase and voice is used is that Festums originally found out about humanity by encountering a voyager like probe - on the probe was a recording of a woman asking this question (where it was meant to be 'Is there anyone out there?' question). The Festums did not understand humanity at all and just echoed that exact recording back when trying to find humans to "save". You can see a framed copy of this record hanging up in the UN leader's office in a few scenes in Exodus.
Second, it's important to understand that Festums, at least in the first season, did not understand humanity at all. (In the movie, they learned a little... which made the situation for humanity even worse, not better). Festums do not exist in the same way that humans do. They do not understand the concept of existence as humans do. They perceive the human's state of existence as an irregularity that needs to be saved from. As they understand so little, the question becomes very literal - they are asking "Do you exist?" - and if someone answers, the Festums become fully aware of the existence of the human that answered and try to "save" them by assimilating them.
This question really is the core of the show's setting. These two races are so alien to each other that this strange roundabout question is the only way they could initially even communicate, and both sides think the other is in a horrific, incomprehensible situation. Humans see assimilation to non-existence as basically death, and Festums see human's existence as something they need to be saved from. Most of the main cast want to establish meaningful communication with the Festums, but at this point, it is unclear if it is even possible, at least without fundamentally changing the nature of the Festums or humans.
