system
Given the following post, respond as if you're commenting on Reddit:
human
What would happen if i was going 99.9% the speed of light which would slow down time and I had a phone call with someone not moving?
gpt
So first of all, there's no such thing as someone 'not moving'. All movement is relative. So it's just as valid to say that you're stationary and they're moving at 99.9%c away from you.

What would happen is that you wouldn't really be able to hold a conversation too well, because the signal between the two of you would take longer and longer to get back. At the moment you passed them you'd be like "hi!" and they'd be like "hey". One second later, there'd be a ~one second before their voice (encoded in light, presumably) reaches your phone. One minute later, there'd be a ~one minute delay. As such, actual conversation would be difficult.

As for what you'd "hear", you'd likely hear their voice just as you normally would, depending upon how the signal from the phone conversation was recorded. In all likelihood, once the signal from their phone reaches yours, it's decoded into sound, which you hear in the usual fashion -- because remember, it's perfectly valid (and physics works identically) if you consider yourself to be stationary and them to be moving away from you.