FTM appears on my screen and the phone is dead?
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It means Field Test Mode.
The most common use of Field Test Mode is to conduct a special kind of field test known as a site survey. If you find yourself living or working in an area with weak signal strength on your device, and you want to boost your signal, performing a site survey will be useful before you choose a signal boosting system. The site survey involves taking several accurate readings of signal strength in and around your building, in order to calculate whether a cellular boosting system will help, and if so, what sort of equipment will be necessary.
Field Test mode is an immensely useful tool in such a situation - switching from unreliable signal bars to a precise numerical value is essential if you want to quantitatively assess your smartphone’s signal strength in different places.
Although a site survey field test is a multi-step process, it isn’t overly complex, and it’s possible to follow the steps and conduct your own test in and around you home or office, using the field test mode on your phone.