That is more than likely because she is using wi-fi calling and iMessage (Apple exclusive text service) it can run off internet connection. Most iPhones have this setting turned on automatically.
Samsung phones do offer Wi-Fi Calling you just have to find it in the settings, this would be a good workaround until the providers get their mess together.
I may be wrong, but regardless I do believe it's a carrier issue because the service outage website I use for AT&T is lit up like Christmas. Make sure you like the accepted solution on page one from the moderators though, I think they are trying to track how many are being affected.
No it's not. My S24 Ultra is working fine. I called my mom today just to make sure. I have Verizon. My girlfriend has AT&T and she can't make or receive calls.
