Every country that has Xbox Cloud Gaming is getting the phone option, but only on supported TVs. On the phone side, you need a compatible iPhone or Android handset with a browser. On the TV side, you need a set that runs the Xbox app.
Here is the honest gap: Xbox has not published a complete list of the TVs, phones or games that work with this preview. It does keep a list of verified TVs for cloud gaming in general on its support site, and that is the best place to check your model.
For background, in January the Xbox app was already on LG’s webOS TVs and Amazon’s Fire OS devices. In January, Microsoft said select Hisense and V homeOS televisions would get the app later in the year, and TCL said then that its Google TV-based X11L series would add it in a future software update. Xbox also said at the time that Game Pass members on the Ultimate, Premium and Essential tiers can stream games through the app on supported TVs.
Microsoft’s own warning is worth taking at face value. It says both new experiences are “still being tested,” and that games and devices may be limited while the preview runs. No date has been given for a full release.