Apple highlighted the difference on the technical specification page for the iPhone 7.
Models A1778 and A1784 do not support CDMA networks, such as those used by Verizon and Sprint.
The reason these models don't support CDMA is likely Apple's decision to diversify suppliers using Intel for the modem in the AT&T and T-Mobile models and Qualcomm for the modem in the other models.
Re/code confirms this is likely the case, "Sources familiar with the situation as well as outsiders agree that the modem diversification effort is the likely reason for the separate phone versions. Apple and Intel declined to comment on the matter, and a Qualcomm representative was not immediately available for comment."
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