Why can't my iPhone without a sim card make and receive Wi-Fi calls like my other iCloud connected devices?

I recently upgraded to an iPhone XS max and I removed the sim from my iPhone 6S Plus relegating it to iPod status. Because I'm with a carrier that supports wifi calling on icloud supported devices I thought that I'd be able to make and receive calls on my old iPhone (through facetime) even if my new iPhone is turned off. All of my other devices can use this feature just fine, I can make and receive calls without my primary phone being present or turned on with no problems, but the old iPhone doesn't do that and I'm not sure why. I went through all of the steps; both phones are signed into the same apple id, on my new iPhone in the "Calls on Other Devices" screen I set the toggle next to the name of my iPhone 6S Plus to allow it to receive calls, in the wifi calling screen of the new phone I switched the toggle on for "Add Wi-Fi Calling for Other Devices", on my old phone I signed out and signed back into facetime and I'm confident that my Wi-Fi network is working properly. What I don't understand is that if my old phone is regarded as an icloud device and if I selected, "Allow Calls on Other Devices" and "Add Wi-Fi Calling for Other Devices" two features that are specifically intended for devices that are signed into my icloud account, why can't my old iPhone work like the other devices?