I think I knew the problem ?
I have 2 iphone 6,
1st one, T Mobile, got it from Ebay, was locked to TMobile first, then told them I was going out of country, asked TMobile to unlock it for me, they did, and it works, it did not show " activation required" problem.
2nd one, Sprint, got it from Ebay, it's unlocked, but it keep saying " activation required " from time to time,
I believed, this iphone was not unlocked from Carrier or Apple, it was unlocked from one of " Unlock Service " from Ebay.
My guess, the one showing " activation required " was unlocked from a "bypass unlock", not a real unlock, so it need to be unlocked time after time,
not like the real unlock, such as my TMobile mentioned above, it was unlocked from my itune ( unlocked from Apple Company ), was a real unlocked from deep inside the iphone.
Questions to all who have this problem,
Who is your carrier ? does this only happened on Sprint ?
Was your iphone purchased from Ebay ? was unlocked from Carrier ( then connected to itune to unlock it ) ?