How to Fix “Connection Problem or Invalid MMI Code”

The “Connection problem or invalid MMI code” message means your phone couldn’t run a dialer code, such as *#06#, *#21#, or a carrier balance code. Most of the time the fix is to retype the code exactly, send it from the right SIM, toggle airplane mode, or restart the phone. If the message keeps coming back, the code probably isn’t supported by your carrier or plan.

These fixes are for Android phones, with Settings paths for Google Pixel (Android 16) and Samsung Galaxy (One UI 8) as of October 2026. Other Android brands have the same settings under slightly different names. The exact wording of the error comes from the Android dialer.

What an MMI Code Is

MMI stands for Man-Machine Interface. It’s the name for the short codes you type into the phone’s dialer that start with * or # and usually end with #. They fall into two groups:

• Phone codes run on the handset itself. *#06#, which shows your IMEI number, is the best-known one.
• Carrier codes (also called USSD codes) are sent to your mobile network. These check a prepaid balance, turn call forwarding on or off, or show your data usage.

The error appears when a phone code isn’t recognized or when a carrier code can’t reach the network or isn’t accepted there. Your normal calls and texts often still work fine.

Fix 1: Retype the Code Exactly

A single wrong character is the most common cause.

1. Open the Phone app and tap the keypad icon.
2. Clear anything already in the number field.
3. Type the code exactly as your carrier or phone maker lists it, including every * and #. Most carrier codes end with #.
4. Tap the green Call button. Phone codes like *#06# run as soon as you type the last character, without pressing Call.

Get the code from your carrier’s own website or app. Codes shared online are often for a different carrier or country and won’t work on yours. If the code is something like *#21# for call forwarding, a US carrier may use a different format. For example, our guide to call forwarding on an iPhone shows the settings menu route that avoids codes entirely.

Fix 2: Send It From the Right SIM

On a dual-SIM phone, or one with both a physical SIM and an eSIM, the code has to go out on the line that belongs to the carrier you’re asking. A balance code sent through the wrong line gets rejected.

• When you tap Call, the phone may ask which SIM to use. Pick the carrier the code belongs to.
• Pixel: Go to Settings > Network & internet > SIMs, tap the carrier, and check that Use SIM is turned on.

• Galaxy: Go to Settings > Connections > SIM manager. Check that the SIM is turned on, and under Calls, choose Ask always so you can pick a line each time.

Fix 3: Toggle Airplane Mode

This forces the phone to reconnect to the mobile network, which clears a lot of short-lived connection errors.

1. Swipe down from the top of the screen to open Quick Settings.
2. Tap Airplane mode to turn it on.
3. Wait about 15 seconds, then tap it again to turn it off.
4. Wait until signal bars return, then dial the code again.

Fix 4: Restart the Phone

Google recommends a full power-off rather than a quick restart when the network misbehaves. Press and hold the power and volume up buttons, tap Power off, wait 30 seconds, and turn the phone back on. Try the code once the signal bars come back.

Fix 5: Check Your Signal and Wi-Fi Calling

Carrier codes travel over the cellular network. If you have weak or no cell signal, or you’re relying on Wi-Fi calling, they can fail even though calls still work.

1. Turn off Wi-Fi so the phone uses only the cell network.
2. Check the signal bars in the status bar. If they’re low, move near a window or outside.
3. Dial the code again.

If the phone shows no service at all, that’s a broader problem. Our guide on fixing an Android SIM that’s out of service starts with the Wi-Fi calling setting, which often causes it.

Fix 6: Check the Network Mode

Some carriers handle certain codes better on their recommended network setting. If you changed it in the past, set it back.

• Pixel: Settings > Network & internet > SIMs, tap the carrier, then Preferred network type. Pick the 5G or LTE option your carrier recommends.
• Galaxy: Settings > Connections > Mobile networks > Network mode, then choose the automatic option at the top (such as 5G/LTE/3G/2G (auto connect)).

Fix 7: Ask Your Carrier If the Code Is Supported

If every fix above fails and only this one code is affected, the code most likely isn’t active on your line. That’s common when:

• You’re roaming in another country. Many balance and forwarding codes only work on your home network.
• Your plan doesn’t include the feature, like call forwarding or a prepaid balance check.
• The carrier changed or retired the code.

Contact your carrier, give them the exact code and error, and ask whether the feature is turned on for your line. Most carriers also show the same information (balance, usage, forwarding) in their app or account website, which skips the code entirely.

If Calls and Texts Also Fail

When you can’t make normal calls either, the code isn’t the problem. Work through these in order:

1. Check that the SIM is active (see Fix 2) and that airplane mode is off.
2. Remove the SIM tray, check the card for damage, and reinsert it.
3. Update the phone: Settings > System > Software updates on a Pixel, or Settings > Software update > Download and install on a Galaxy.
4. Reset mobile network settings. On a Pixel, go to Settings > System > Reset options > Reset mobile network settings. On a Galaxy, go to Settings > General management > Reset > Reset mobile network settings. This clears network preferences but doesn’t delete your photos, apps or contacts. See Google’s Pixel connectivity help and Samsung’s call troubleshooting page for the full lists.
5. If none of that helps, call your carrier from another phone. Your account may be inactive or there may be an outage.

For more call problems, see our guide on what to do when an Android phone won’t make calls.

Frequently Asked Questions

Does “invalid MMI code” mean my phone is broken?

No. It only means one code didn’t work. If you can make a regular call and send a text, your phone and SIM are working.

Why does *#06# show “invalid MMI code”?

That code should work on almost every phone, so first make sure you typed it into the keypad of the built-in Phone app and not a messaging or third-party dialer app. If it still fails, you can find the IMEI in Settings > About phone instead, as shown in our guide to finding a phone’s IMEI number.

Will adding a comma to the code fix it?

Usually not. A comma tells the dialer to pause before the next characters, which is useful for extensions but not for carrier codes. Retyping the code from your carrier’s own instructions is a better use of your time.

Can I use carrier codes while traveling abroad?

Often not. Many carriers only accept balance, data and forwarding codes on their home network. Use the carrier’s app or website while you’re roaming.