This phone has been GSM unlocked.  It can function on either GSM networks (such as AT&T and T-Mobile) or Verizon's CDMA network.  It can't be used on Sprint network (including Sprint, Boost Mobile, Virgin Mobile or any Sprint MNVO carrier). It can't be used on US Cellular network either.

1) For GSM carrier customers: this phone is compatible with any GSM network.  Ideally, the phone should work after you insert your own valid SIM card and power on the phone.  But in some situations if the automated process doesn't work, you may need to manually enter the correct APN (access point name) obtained from your carrier and reconfigure the mobile network setting in order to make the voice*, text and data work in your carrier.  There are lots of YouTube tutorial videos available for your reference. 

*A trick to make/receive phone calls with your own GSM SIM card inserted: if the voice function doesn't work after you power on the phone, please go to Cellular Network Settings and switch the Preferred Network Type from LTE to 3G or 2G, you should be able to make/receive phone calls. You might switch back to LTE later and still have the voice function work.

2) For Verizon (CDMA carrier) customers: this phone can only be used in Verizon Prepaid service right out of the box.  If you want to use this phone on Verizon postpaid service, or any Verizon's MVNO providers such as Page Plus, Tracfone, Straight Talk, etc., you must use this phone on Verizon Prepaid for a year or until $75 in payments are received by Verizon prepaid service (which can be achieved in 2-3 months).  You need to activate the phone on Verizon Prepaid and choose a monthly plan. After the prepaid commitment is fulfilled, you can switch to your desired CDMA carrier (please make sure you have a valid 4G LTE sim card and follow the activation/switching service instruction from your CDMA carrier).