Did you know: some Sony Xperia smartphones have hidden FM transmitter functionality
39

While this post is dedicated to a certain hidden feature in some Sony Xperia smartphone models, we'd like to start with a jump back in time – to 2007, more specifically, when the Nokia N95 was arguably the best smartphone in the universe. It had a spacious display, GPS and Wi-Fi connectivity, and a great (for the time) camera, among other noteworthy goodies. But it also offered a feature you won't find on any recent smartphone, be it an iPhone, an Android handset, a Lumia, or a BlackBerry. The feature in question is a built-in FM transmitter. It allowed one to broadcast the music playing on their N95 and play it on their car's stereo by tuning the radio to a specific frequency. No wires required!
As we already stated, today's smartphones don't have the ability to broadcast audio over the FM band. At least not out of the box. It has been discovered, however, that a hidden FM transmitter feature can be enabled in a handful of Sony Xperia phones as their hardware allows it, at least technically. The Sony Xperia Z, Xperia Z Ultra, Xperia Z1, Xperia Z1 compact, Xperia T, and Xperia SP are all on the list.
source: XDA Developers
Things that are NOT allowed: