If you are experiencing dropped calls or sluggish cellular data speeds, looking at the tiny “bars” in the top corner of your iPhone screen provides almost zero actionable information. Three bars on one network might represent a vastly different signal quality than three bars on another network.
To see the exact, raw numerical data regarding your cellular connection—including precise signal strength, frequency bands, and tower telemetry—you must access a hidden engineering menu built into iOS called “Field Test Mode.”
How to Enter Field Test Mode
Apple hides this diagnostic tool behind a specialized dialer code.
- Turn off Wi-Fi on your iPhone to ensure you are actively using the cellular network.
- Open the default Phone app.
- Tap the Keypad tab at the bottom of the screen.
- Carefully dial the following exact sequence of characters and numbers:
*3001#12345#*
- Tap the green Call button.
Instead of placing a phone call, your screen will instantly transition into a grey, highly technical dashboard labeled “Field Test.”
How to Read Your Signal Strength
The Field Test interface varies significantly depending on your iPhone model and the specific modem (Intel or Qualcomm) installed inside it. However, the metric you are looking for is universally named RSRP (Reference Signal Received Power).
- On the main Field Test dashboard, look for a tab or menu item labeled LTE or 5G (depending on your active connection).
- Tap on Serving Cell Meas (Serving Cell Measurements).
- Locate the metric labeled rsrp0.
This number represents your true signal strength, measured in decibel-milliwatts (dBm). It will always be a negative number.
- -70 dBm to -90 dBm: Excellent signal. You are likely very close to a tower.
- -90 dBm to -110 dBm: Good to fair signal. This is standard for most urban environments.
- -110 dBm to -125 dBm: Poor signal. You will likely experience dropped calls and extremely slow data speeds.
You can actively walk around your house or office while watching the RSRP number fluctuate to determine the exact location with the strongest cellular reception.
To exit Field Test Mode, simply swipe up from the bottom of the screen (or press the Home button) to return to your normal iOS home screen.