How to link iphone to another apple id

Hello all, my dad is a undercover and his boss gives him and his coworkers work phones (iphone 4-6). His coworker, Peter forgot his pass code to his iphone 5 back in March 2016. My dad said I could have the phone so I resetted it with my iTunes account but it won't let me get past the activation lock because the phone is linked to Peters account. I asked peter but he said he had forgot it. Any help?? Btw, I'm new to Apple.