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I have always assumed that when I make a voice call from the FaceTime app on my iMac, the call is being proxied through my iPhone. (In fact, the FaceTime UX says, Call Using iPhone; see attached image.)

enter image description here

However, today I tried an experiment: I switched off both my iPhone and my iPad--which has a cellular plan; I also turned off the WiFi on my iMac so that it was connected to the Internet only through an Ethernet cable to my router. I then used FaceTime on my iMac to call my landline. The call went through, reaching the answering machine hooked up to that phone.

My question is: How did this work? Does my iMac have some sort of VOIP system that it can use if it can't access a cellular device?

I am running macOS Big Sur 11.6.5 (20G527)

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Yes, that is correct. Apple calls this Wi-Fi Calling.

The feature requires support from your carrier. The carrier has an Internet-accessible VoIP system that your iPhone can use when it has an internet connection, but it has no cellular connection (or very low signal strength) - and similarly it can be used by your other devices, such as the Mac, if you have enabled that.

Note that even though it has the name "Wi-Fi Calling", which seems to be confusing to some, the technology has absolutely nothing to do with Wi-Fi and does not require you to use Wi-Fi. If your Mac is connected to the Internet over Ethernet for example, that works just as well.

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  • And this follows the common mode of "everything is WiFi" when in fact, except for peer-to-peer WiFi connections (e.g., connect to a printer directly instead of via a router), nearly all internet connected devices really don't care whether they are connecting via WiFi or wired ethernet. By and large, you can pretend WiFi is a magical way of connecting devices as if they were wired to a router. I actually prefer to connect wired whenever possible even if a device has both WiFi & wired capability - setup is generally easier and connections more reliable. Mar 25, 2022 at 0:57
  • I’m not really sure what that has to do with anything… but yeah, and no. Everything is not WiFi on the internet, but everything is basically IP. That’s why it doesn’t matter if you’re using WiFi, Ethernet, USB, Bluetooth, RS-232 serial or any other method to transfer your data.
    – jksoegaard
    Mar 25, 2022 at 3:17
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    My point is the "Wi-Fi Calling" and similar phrases used by vendors and users that include "Wi-Fi" don't actually need WiFi to work, and in fact often don't know/care whether WiFi is involved or not. It is just such a common misconception out there - "WiFi = magic". Mar 25, 2022 at 3:20
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    @cjs The point Apple (and other vendors have it too) is that it is "your local high speed network" instead of "cellular". Which for the non-geeks means "magical WiFi". Interesting question as far as cellular data. The reality is that for most carriers and most devices in most areas, if you don't have a good cellular voice connection (which is all digital anyway) you aren't going to have a good cellular data connection, and vice versa. But theoretically (assuming Apple doesn't block it), it should be fine. One example would be a phone on one network (e.g., Verizon) used as a WiFi hotspot to Mar 25, 2022 at 4:33
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    provide data service to another phone (e.g., T-Mobile) that doesn't have good cellular service (voice or data) in that location. The T-Mobile phone should then be able to do a VOIP "Wi-Fi Calling" over the Verizon cellular data network. In the end it shouldn't make a bit of difference. Mar 25, 2022 at 4:35

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