Have you heard about “whisperhacking?” Of course not – it’s inaudible to you, but not to Siri and Alexa, and that’s the problem.
Gadgets with microphones can hear sounds at a very high pitch, much higher than humans can. If you had a 500-key piano, the highest keys would be barely audible to you. Siri and Alexa can hear sounds more than twice as high as that.
Its possible for hackers to send commands to your iPhone at such a super-high pitch that Siri will hear and follow them, but you can’t hear them at all.
So if your iPhone starts dialing itself, you might not be imagining it.