Sony shows off its hot new Sony Xperia Z1S on video, complete with T-Mobile branding5
Sony also wanted to point out in the video that the headphone jack is waterproof, just like the rest of the phone and all of the ports. Therefore, even if you're listening on your headphones when you fall into the pool (or get pushed), your Xperia Z1S will survive. This is not a change from the Sony Xperia Z1, which also has a waterproof headphone jack, but is a improvement from the Sony Xperia Z, which is currently available via T-Mobile.
The video also notes that online orders will begin on January 13th, while brick and mortar sales at the carrier begin on January 22nd. Considering what a hot number this phone is, and how innovative T-Mobile has become, the Sony Xperia Z1S and the nation's fourth largest carrier seem to be made for each other.