##3282 or ##3282# were available on some Lumia 1520 on Windows Black; so far, from what I tried Vietnam Rom has this feature but unfortunately 4G is disabled on that rom which made it useless to take advantage faster speeds that 4G LTE can provide.
Native Toast Launcher was an alternative for those ROM that does not support the dial code ##3282+call/ ##3282#. You need to have a developer unlocked phone to be able to deploy the Xap file and install it like a regular application. Unfortunately the Launcher stopped working after the Cyan update 8.1.
I'm making this thread so that anyone who'll be able to develop one (Native Toast Launcher) for this can post or share their knowledge here. Take note this is for Cyan
Please post updates here if your ROM supports the field test still after the Cyan update. Right now I don't know if Vietnam Rom still has this with Cyan update but it was reported that those that had "developer upgrade" to Cyan still has the Dial Code Field Test option, however I would also want to validate if it now supports LTE.
Please Post your "Product Code and Country Region" so that other will be able to flash to that Rom.