I purchased a second hand iphone 6s which is locked

If the iPhone is locked you need to give back the device (and get back your money) or you won't be able to use it.

If you can't contact the previous owner and/or if you don't know his credentials, your device is useless. Sorry.

Regards

Giulio

My son's iphone was stolen and we locked it down. Never got it back either, but I did hear from someone briefly who said he/she bought it on ebay. You probably purchased a lost or stolen phone.