IPhone Field Test Mode. The iPhone has a hidden built in Field Test Mode app. To access it, follow these steps Go into Settings > WiFi and turn WiFi Off. This will enable you to see what network you re connected to G, G, G, LTE, etc . For iOS . and above Go into Settings > Cellular > Cellular Data Options > Enable .It s often useful to know how strong a signal your iPhone is receiving. If you re trying to make a call but it won t go through or if you re otherwise not getting the cell service you expect, then knowing your signal strength can help you determine why not. Of course virtually all phones have the “bars” or “dots” .Check your Apple warranty status. Enter a serial number to review your eligibility for support and extended coverage..