SIM Network Unlock Codes

May 29, 2018
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I came across à ZTE Grand X4 Cricket Phone that I'm wanting to switch my service to. It requires a SIM's card just like my AT&T phone I'm currently using. So I decided Since my AT&T SIM's card would fit the ZTE Grand X4 phone & I'm wanting to keep my same service provider & phone number then I just put the card in and see if would work. But when I power up the Cricket ZTE Grand X4 phone with my AT&T SIM's card in it then it wants me to enter a SIM Network Unlock Code. So my question is how do I obtain this code and still be able to use my AT&T SIM card for my new ZTE Grand X4 phone?