i have an i5s that is locked by a smaller Alaskan network carrier. My contract is paid off with them no longer use their service. How can I obtain an unlock code for my phone legally

i have an i5s that is locked by a smaller Alaskan network carrier. My contract is paid off with them and no longer use their service. How can I obtain an unlock code for my phone legally so I can use it with another network carrier? I have tried obtaining an unlock code through a few "cell unlocking" services but they all seem to find themselves lost and unable to acquire a code. Am I completely out of luck and should accept the fact that it is unobtainable?