Installed Steam on a new computer. Signed in. It sent a passcode to my GMail. I signed into GMail. It wanted me to 2FA because I hadn't signed into Google on that device. It sent a notification to my phone, which I never received. I had it resend the notification twice, still nothing. Tried again with my phone's offline passcodes. Neither worked. Tried the QR code/Bluetooth connection, and that finally did it.
At least I got through in the end, but fuck, it's annoying.
VoIP is not supported for 2FA by some institutions like banks because it may be less secure than a conventional phone line, since it is connected to the internet. In practice, I think SMS is insecure regardless whether it is over the internet or phone line, but in any case that is why VoIP is not fully supported.