Hello, Robti.
Sounds like you have it right. Even without any network signal, Wifi calling can step in to allow you to make and receive calls, and send and receive texts. I'm unsure why it wasn't working for you in this case, but Trevor has made some good suggestions on what you could try in order to figure it out. I'd also add to that, try putting the phone into airplane mode, then turn Wifi back on while in airplane mode, then try a test call.
Pete.
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