with other carriers, you can use your iPhone to make calls over the wifi network , and use it whenever you feel like it or whenever cellular reception is low, AT&T is working on it.
with t-mobile's wifi calling your iPhone # is shared via iCloud, and any device can be used to make and receive telephone calls over the wifi network , when the iPhone is not nearby, or when it is even turned off
https://support.apple.com/en-us/HT203032
i bought the stainless steel one, because of the better sapphire screen and ceramic back.
the regular aluminum one has a gorilla glass type screen and a composite black ( plastic )
the ceramic back looks skinnier and higher quality then the composite back, which looked dull to me