Can’t fix ‘iPhone calls not available, your iPhone must be signed…… make calls from this iPad.’

I had connected my iPad Pro(Wi-Fi model) to my iPhone 12mini just yesterday to receive and call from iPad, it worked then but after a day it won’t call or receive any call saying that ‘iPhone calls not available,…’ during that one day period I disconnected my iPad from the Internet and turned off Wi-Fi and Bluetooth on iPhone (it was turned on on iPad). FaceTime is working on both though, I just can’t receive or call from iPad. I tried signing out and signing back-in in FaceTime from both the devices, I tried resting the network settings on iPad, I tried flight mode on both of the devices, I connect them on hotspot and on a same Wi-Fi network. They are logged in with the same Apple ID. How do I fix that.