When you purchase an iPhone under a line you have on your account the sim & phone number is attached to the IMEI # of the iphone.
So when Fedex or UPS delivers the package and when you turn the phone on it will ask for your last 4 social security number this is to verify that no one stole the phone without the last 4 that phone is useless no one can use it or activate it, once you put in the last 4 it will activate the phone just take out the simcard and you can sell the phone.
I sold a jet black 7 plus last year guy calls me asking me for my last 4 of social i did not believe him at first but I told him lets meet him and he was actually right the phone was asking for the last 4 of my social for him to bypass the activation screen and put in his own simcard.
However, if you purchase the SIM FREE UNLOCKED phone from apple which is not available at the moment you wont run into this issue.