samsung galaxy S4 unlocked asking for SIM network unlock pin

Hi guys

I recently purchased an S4 from an on-line professional seller, the guy apparently owns a physical store but sells some phones on-line

when the phone arrived, I turned it on without the SIM, insert my home's wi-fi connection password, and the phone started to update both apps, and the OS to lollipop

I installed the backup I previously got from my old s3 mini, installed all my apps from playstore, and once I installed whatsapp, I turned the phone off, and than on again, this with my SIM since whatsapp has to confirm my number

The phone accepted my PIN code, whatsapp accepted my number and restored it's backup, but the phone also asked for this SIM network unlock pin

I told the guy what's happening, he told me he would send me a new phone, but I suspect this can go wrong

If that's the case, can anyone here tell me how can I overcome this SIM network unlock pin issue ?

thanks in advance

Regards
Antonio