Comment by chatmasta

If you have it enabled, then every thirty seconds (regardless of whether you’re actively on a call), your phone will make a request to a signaling server owned by your mobile ISP. So if you’re on T-Mobile and traveling in some other country with no cell service, but you’re connected to WiFi, then T-Mobile will see your public IP address. (IIRC, this also bypasses any VPN Profile you have enabled on your device, because the signaling system is based on a derivative of IPSec that could have problems communicating over an active VPN tunnel.)

I found out about this when I was wiresharking all outbound traffic from my router and saw my phone making these weird requests.

Apple actually does warn you about this in the fine print (“About WiFi calling and privacy…”) next to the toggle in Settings. But I didn’t realize just how intrusive it was.

I know my mobile ISP can triangulate my location already, but I don’t want to offer them even more data about every public IP of every WiFi network I connect to, even if I’m not roaming at the time.