You need to contact your carrier to change the number. If he owns the account - you need to branch out and find your own service, then do another reset on your phone (signing in with an account he does not know, even if that means a brand new account). When you reset the device, be sure to do so in a way that prevents you from being blocked by Factory Reset Protection -
http://www.androidcentral.com/factory-reset-protection-what-you-need-know
If you need inexpensive service, there are two low-cost carriers that allow you to bring any device as long as it works on the network they run on - MintSIM works on T-Mobile towers and Ting which runs on Sprint towers.
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