*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.
Definately, I looked at a number of sources including nextworth, Gazzell, and the Apple store before upgrading my wifes iPhone last week. Apples value was way above what the other two sites were and I have dealt with nextworth many times in the past.
Look up your numbers on both of those sites, check out both ATT's options as well as Apple then go with the best one. Prices fluctuate so what was good for me may be different today. There is no cost to determine your current value.
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Information on iPhone unlocking can be found here by copying this link into your browser: https://www.att.com/deviceunlock/client/en_US/