I am using samsung galaxy s6 since september
Here I want to give you the important information before buying second hand cell phones specially s6 or s7
Before buying second hand phone first check the imei number of the phone if it shows the imei number
Than go to settings > about > model number
And check if the model number contain p ( ex. SM-G920p) at the end of model number than it is sprint phone
About the sprint phone :
Sprint is company which provide free cdma cell phones on buying sim card
In sprint phone you can only use cdma sim and some mobile shopes are importing sprint phones at the garbage price and selling it by unlicking it temporarily
And if you update sprint phone then phone will be cdma again and u will regret buying that phone soo must check or ask isnt it a print or verizon phone or not?
& if you have a phone of that format or model number than please dont update your phone ever or you will have to pay the cost for unlocking it again
If you have any query than ask in comment