The iPhone 6s and iPhone 6s Plus come in three different models each. Apple has mostly consolidated the needed LTE wireless bands to make their products compatible with the latest network technologies. However, there are still differences between the frequencies supported with each model number.
Here are the basics when it comes to each iPhone 6s and iPhone 6s Plus model number:iPhone 6s A1633: SIM free (unlocked), AT&T, Mobile Virtual Network Operators (MVNOs)
iPhone 6s A1688: Sprint, T-Mobile, Verizon
iPhone 6s A1700: China Mobile, China Unicom, China Telecom
iPhone 6s Plus A1634: SIM free (unlocked), AT&T, MVNOs
iPhone 6s Plus A1687: Sprint, T-Mobile, Verizon
iPhone 6s Plus A1699: China Mobile, China Unicom, China Telecom
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When it comes to the difference between iPhone 6s A1633 and A1688 (or the difference between A1634 and A1687 on the iPhone 6s Plus) it comes down to one band. The SIM free / AT&T models support 2300 Mhz (2.3 Ghz) otherwise known as Wireless Communications Service (WCS) or band 30, and the others do not.
While A1688 and A1687 models will also work on the AT&T network, A1633 and A1634 are the only ones that can utilize band 30. AT&T is the only carrier with WCS capability on its network. Opening up this 2300 Mhz frequency makes it possible for AT&T to provide users with higher LTE speeds and additional capacity in densely populated areas.