What is the request number for a device?

I was given an iPhone 5c by a friend recently, he was originaly with at&t so his phone is programed with that frequancy. I use a verizon wireless plan and im trying to unlock my phone so as to use it with verizon, however, when prompted to state a IMEI number i did so but there as another feild required to be filled out, the request number. Im curious as to how to find this number for a specific device so i might use my phone. Help!!!