FAQ:

LineageOS removed builtin root, so you need to flash the root package linked above.Reboot. Data doesn't work on the first tryReboot. Then, make sure your APN settings are correct.Credits to @ paulq75 Settings -> Apps -> tap the cog icons up to the right -> and the set the default Phone app.Usually happens when switching to/from TouchWiz roms. Solution would be to backup your data to PC and format internal storage. Sometimes even multiple times.Known bug. Happens only on first attempt. Solution would be to close the app, remove from recents and start again.Make sure you are using the latest TWRP version from this thread.See Reporting Bugs below