I’m curious about how this works, can you link me someplace where I can read more about it? I tried searching for “cellular over wifi” but wasn’t about to find anything promising.

EDIT: I searched harder and found “VoWiFi”. It looks like this can support SMS and is supported by iOS. TIL.

For someone more curious, I tried to learn more about this tech, and this [1] is the best resource I found about someone explaining how it works.

Essentially VoWifi connects to your ISP via a IPSec VPN using secrets derived using your sim card as a smart card, then connects to a internal SIP server and voila.

I spent a lot of time trying to connect to the IPSec server from a computer, but couldn't figure out how to get the challenge solved by my sim. Gosh I wish I could make normal calls from my computer.

1. https://worthdoingbadly.com/vowifi/

Thanks for this link! This is a great explanation which clears a lot of things up to me.

Bingo. My carrier offers it as Wifi Calling. It worked surprisingly well.

4G is not so bad here because of the 700 / 800 MHz band which has really good penetration.

But 5G is mainly done on 3.5Ghz and that's tough yes. Of course they can use the same frequencies for 5G. Though the bandwidth at such low frequencies is limited, and thus the maximum speeds.