iPhone's IMEI Issue

Is there any way I can unlock my iPhone's IMEI? The carrier (Net10) I've been using for five years and Net10 refused to unlock the iPhone that I paid fully unlocked from Best Buy when I tried to transfer service to a new SIM card for a better data. I've tried to contact both old and new carriers to work this out for three days now and still couldn't do it.

Any suggestions to have IMEI unlocked? I still have the receipt as well as it said it is supposed to be an unlocked iPhone.