11-01-2017 04:06 PM
The reason for this is due to Apple's US Reseller Flex Policy. You need to insert a US-based carrier SIM card in order to activate the phone for use outside the United States. I would recommend inserting a Verizon SIM card which will unlock the device fully for international use. Using another SIM card from a different carrier will lock the iPhone to their unlock policy, hence why I recommend Verizon.
This needs to be done in the United States, however. Afterwards, the phone will be capable of international use.