I tested about 20 iPhone 6S & 6S Plus on iOS 9.2.1 to 9.3.2. If I erase all content and settings and return to the activation screen, the iPhones require me to restore to the newest version in iTunes to bypass the "Activation error' message. If i request a carrier unlock code and then try another SIM card for an already activated iPhone, I get the "Activation error" message and have to update as well. Anyone on iOS 9 want to test this and report back? Any way for Apple not to force us to update to the newest iOS?