When it comes to Custom ROMs, CyanogenMod is probably the first one that comes to mind. Simply because it was one of the first, it supports the most devices, and it offers a ton of customization options while still being stock Android. Since CyanogenMod started, a lot of things have changed. Including them turning into a company, named Cyanogen Inc., and working with various smartphone makers around the world to bring their version of Android to more and more users. They currently have partnerships with ZUK, WileyFox, and many others. CyanogenMod, however, has remained very popular. And is now working on Android 6.0 Marshmallow, since Google has made the code public.
CyanogenMod 13.0 will be based on Android 6.0 Marshmallow, and will include most of the staple CyanogenMod features, like the theme engine, the customized quick settings and much more. But there's something else being added to CyanogenMod 13.0, and users on T-Mobile US will love this feature. WiFi Calling. T-Mobile has had WiFi Calling for quite some time, but for the most part it was only for their own devices. Devices they sold, with the exception of the Nexus. But recently, T-Mobile opened that up to any smartphone that would work on their network. So more and more can use WiFi Calling. It appears that CyanogenMod is using some code for WiFi Calling that is in AOSP as well as some of their own trickery from Cyanogen himself, Steve Kondik, to get T-Mobile WiFi Calling available in CyanogenMod 13.0.
Roman Birg, from CyanogenMod, posted the screenshot you see above which shows T-Mobile WiFi Calling. this is great news for anyone on T-Mobile looking to flash CyanogenMod 13.0. However it appears that this won't work for Sprint's WiFi Calling just yet. However that could change in the future. CyanogenMod is working hard on CyanogenMod 13.0, but as you probably already know, there are no OTA's available for when the first nightlies will begin to roll out. They have had access to the code in Marshmallow for a few weeks now, so hopefully it won't be much longer. Additionally, it's fairly likely that Nexus devices will see nightlies for CyanogenMod 13.0 before other devices.