: Open the Phone app and dial *3001#12345#* , then press the call button.

: There are technical guides on how to make the FieldTest application icon permanently visible on jailbroken devices. 2. Starfinder & Pathfinder RPG (Paizo)

Several brands and review sites use the term for real-world equipment testing: Fieldtest - PenCott Metropolis (urban camouflage) - part 1

: Users on Reddit r/cellmapper often post updates about changes to this interface in the latest iOS betas.

If you are trying to find the hidden menu on your iPhone to check signal strength or LTE bands: