How do I make WiFi calls on my iPad when the IPhone is not accessible?
This depends on support from your carrier. Per the document you linked:
Note: Wi-Fi Calling on other devices is available with some carriers, and cellular charges may apply.
To enable other devices:
- Tap Wi-Fi Calling, then turn on Add Wi-Fi Calling For Other Devices.
This allows iPad and other devices where you’re signed in with the same Apple ID to make and receive calls even when your iPhone isn’t nearby.