Hmm that's extremely odd that it happened out of the blue, priyapam. If you're disconnected from WiFi, do calls go through or are you only having issues with texts? It sounds like iMessages are working probably and maybe WiFi calling is working for you too. Do you happen to have a different T-Mobile phone to test your SIM in?
Good morning, priyapam!
I just wanted to swing by to see how things are going with your phone today. Are you still receiving errors related to the iPhone not being activated or is everything good to go now?
I have the iPhone 6S and am having this problem as well. It shows that wifi and phone service are on but I cannot access anything at all. It opens with "hello" which I must swipe and it asks for my password as I keep it locked. I put that in and get the message: "Activation could not be completed. Please restore the phone and install the latest version of iOS and attempt actuation again. If the problem persists, please contact customer support."
One page on Apple discussion suggested removing sim card and putting back in. It did not help. Funny thing is when I attempt to turn on the phone the opening wallpaper shows for a flash of a second and then the "hello" comes on.