Check if the phone is compatible with the carrier:
Please scroll to "Check Compatibility" on these websites and type the imei.
These websites will tell you if the phones are compatible. Use this for your cdma/gsm unlocked devices.
2) Boost Mobile compatibility . Boost mobile does not accept s8/9/10s at this moment even if the site shows that they are compatible.
Check if the phone is on the "Blacklist"
Select US Black list and type the imei. If it says "FAILED" on the blacklist, the phone is reported lost/stolen.
Pro Tip: If the phone is NOT unlocked, after checking for compatibility, put the sim card for the carrier in the device, connect to wifi. If you get an error "Sim not supported", or a message to unlock the device, device MUST be unlocked first before it would work for the carrier.
GSM unlock means the phone will work for GSM carriers in the US and Internationally (ATT, Tmo, Cricket, H2O, Boost, Metro, Simple Mobile)