It's just a regular 4G phone. They got LTE base stations up on the satellites. Actually, I'm not that surprised that they got the downlink working but I also would love to learn abut the challenges they faced while trying to receive a clean signal from a regular cellphone 600+km away.

Cellphones' transmit power is severely limited due to a) proximity to human brain b) battery considerations.

I imagine it also requires a clear view of the sky. That's how any satellite phone tech I ever laid hands on worked. I'd be astonished if they got it also working indoors. That'd be just pure magic.

https://www.starlink.com/business/direct-to-cell

https://arstechnica.com/gadgets/2023/10/spacex-details-starl...

EDIT: Just to be clear, the link says nothing about the return channel but I imagine they wouldn't have made this announcement unless they got the return channel working as well.