Q: is there a default passcode to reset iphone 6

I received a warranty replacement iPhone6.  Of course, just trying to get it loaded and restored is already 20 problems.  The only security code I have entered on it is the six digit required one to start the phone. Yet,when I get to factory reset [which the tech wanted to try just in case, since it says I don't have enough "room" on my 128gb phone to restore to a previous back up on the SAME phone, which makes no sense], it requests a 4 digit password to rest the phone. Since I never set one, I tried just putting one in to set it, and it said it was the wrong one.  But I have NOT used any other password on this phone!  Is there a default password that comes with them for the "reset" feature, or did they send me a refurbished phone with a lock already on it so that I will never be able to reset again?