How To Fix Connection Problem or Invalid MMI Code Error

The “Connection Problem or Invalid MMI Code” error pops up on Android when your phone fails to process a USSD or MMI code you entered in the dialer. The most common cause in 2026 is a network connectivity glitch, a dual-SIM routing conflict, or your carrier dropping legacy USSD support as they migrate to VoLTE. Below are the most effective fixes, ordered from the quickest and most common solution to more advanced options.

What MMI and USSD Codes Actually Are

MMI stands for Man-Machine Interface. Any code you punch into your phone dialer that contains an asterisk (*) or hash (#) is technically an MMI code. USSD (Unstructured Supplementary Service Data) codes are a subset — they get sent to your carrier’s network to do things like check your prepaid balance, activate call forwarding, or query voicemail settings. Your phone processes some MMI codes locally (like *#06# to display your IMEI), while USSD codes require a live connection to your carrier.

Here are some of the most common codes that trigger this error:

Code	Function
*#06#	Display IMEI number
*#*#4636#*#*	Android testing/info menu
*#004#	Check call forwarding status
##002#	Erase all call forwarding
*31#	Hide/show caller ID
#BAL	Check balance (Verizon)
*225#	Check balance (various carriers)
*611	Customer service (most US carriers)

If any of these return “Connection Problem or Invalid MMI Code,” the fixes below will help.

1. Toggle Airplane Mode (Fixes It for Most People)

This is the fastest fix because it forces your phone to completely disconnect from and re-register with your carrier’s network. A surprising number of MMI code errors come from a temporary network registration glitch that a simple airplane mode cycle clears.

1. Swipe down from the top of your screen to open Quick Settings.
2. Tap Airplane Mode to turn it on.
3. Wait 15–20 seconds.
4. Tap Airplane Mode again to turn it off.
5. Wait for your signal bars to return, then retry your MMI code.

If this doesn’t work on the first try, do it once more — sometimes the re-registration takes two cycles.

2. Add a Comma or Plus Sign to the Code

Some carriers need a brief pause before processing USSD commands. Adding a comma to the end of your code forces that delay.

To add a comma: If your code is *123#, dial *123#, instead. On most Android keyboards, you can get a comma by long-pressing the asterisk (*) key for 3–5 seconds.

To add a plus sign: Try putting a + after the first asterisk. So *123# becomes *+123#. Long-press the zero (0) key for 3–5 seconds to get the + symbol. This forces the dialer to route the code through international dialing logic, which bypasses some local parsing errors.

3. Restart Your Phone

A full restart clears out temporary glitches in the telephony stack that airplane mode might miss. Hold your power button, tap Restart (not just power off), and wait for the phone to fully boot back up before retrying.

On Samsung Galaxy phones, you can also try a soft reset by holding Volume Down + Power for about 10 seconds until the phone restarts. This clears the RAM more thoroughly than a standard restart.

4. Fix Dual-SIM Routing (Very Common on Dual-SIM Phones)

If your phone has two SIM cards, this is one of the most likely causes. Your phone may be routing the MMI code through the wrong SIM slot — especially if one SIM doesn’t have an active plan or has weak signal.

On Samsung Galaxy (One UI):

1. Go to Settings → Connections → SIM manager (or SIM card manager on older One UI versions).
2. Under Preferred SIM, tap Calls.
3. Select Always ask instead of a specific SIM.
4. Now when you dial the MMI code, your phone will ask which SIM to use — pick the one with the active plan.

On Google Pixel / Stock Android:

1. Go to Settings → Network & internet → SIMs.
2. Tap on the SIM you want to use for the code.
3. Make sure it’s enabled and set as the default for calls.

Alternative: Temporarily disable the second SIM entirely. Go to your SIM settings and toggle off the SIM you’re not using, retry the code, then re-enable it.

5. Check and Reset Network Settings

If your phone’s network configuration has gotten corrupted — common after a major OS update or carrier settings change — resetting it can fix the MMI code error.

On Samsung Galaxy:

Settings → General management → Reset → Reset network settings → Reset settings

On Google Pixel:

Settings → System → Reset options → Reset mobile network settings

On other Android phones:

Settings → System → Reset → Reset network settings

Warning: This will erase your saved Wi-Fi passwords, Bluetooth pairings, and mobile data settings. Write down your Wi-Fi passwords before doing this.

6. Update Your Phone’s Software

Carriers and manufacturers periodically push updates that fix telephony bugs, including MMI code processing. Running outdated software is a common cause of this error, especially after carrier-side VoLTE migrations.

On Samsung Galaxy: Settings → Software update → Download and install

On Google Pixel: Settings → System → System update

Install any available updates, restart your phone, and try the code again.

7. Re-Seat Your SIM Card

A SIM card that’s slightly loose or has dirty contacts can cause intermittent network issues, including MMI code failures.

1. Power off your phone.
2. Use a SIM ejector tool (or a straightened paperclip) to pop out the SIM tray.
3. Remove the SIM card and gently clean the gold contacts with a soft, dry cloth.
4. Inspect the SIM for any visible damage — scratches, cracks, or warping.
5. Place the SIM back in the tray, making sure it sits flat and snug.
6. Reinsert the tray and power your phone back on.

If your SIM card is more than 3–4 years old and you’re seeing frequent network issues, visit your carrier’s store and ask for a free SIM replacement. Most carriers (Verizon, AT&T, T-Mobile) will swap it at no charge.

8. Boot into Safe Mode

Third-party apps — especially call blockers, VoIP dialers, or automation apps like Tasker — can intercept dialer commands and cause MMI code errors. Booting into safe mode disables all third-party apps so you can test whether one of them is the culprit.

To enter safe mode:

1. Press and hold the Power button until the power menu appears.
2. Long-press the Power off option until “Safe mode” appears.
3. Tap Safe mode to confirm.
4. Once the phone restarts, you’ll see “Safe mode” in the bottom-left corner.

Now try your MMI code. If it works in safe mode, a third-party app is causing the conflict. Start by uninstalling the most recently installed apps and test after each removal.

To exit safe mode: Just restart your phone normally.

9. Enable SMS over IMS (Advanced)

This option routes SMS and USSD commands through the IMS (IP Multimedia Subsystem) layer instead of the older circuit-switched path. It’s particularly useful if your carrier has recently migrated to VoLTE and legacy USSD routing is broken.

1. Open your phone’s dialer and type *#*#4636#*#*.
2. The Testing menu or Phone information screen should open automatically.
3. Look for SMS over IMS and enable it.

Important notes:

• Not all phones support this code. Many newer Samsung and Pixel devices have removed the testing menu or restricted access to it.
• If the code doesn’t open anything, your phone’s manufacturer has likely disabled it. Don’t try to force it — this is an advanced diagnostic tool and changing other settings in this menu can cause connectivity problems.
• If you see the option for Turn on IMS over SMS, that’s the same thing — enable it and retry your code.

10. Contact Your Carrier

If none of the above fixes work, the problem may be on your carrier’s end. Some specific scenarios only your carrier can fix:

• Account-level USSD blocks: Some carriers restrict USSD services on certain plan types. A support agent can check and remove the block.
• Network profile reset: Your carrier can reset your network profile from their end, which re-provisions your phone on their system.
• SIM card replacement: If your SIM’s authentication is failing, they can issue a new one (usually free at a store visit).
• eSIM issues: If you’re using an eSIM, your carrier may need to re-push the profile.

Carrier support numbers:

• Verizon: 1-800-922-0204 (or dial *611 from your Verizon phone)
• AT&T: 1-800-331-0500 (or dial 611)
• T-Mobile: 1-800-937-8997 (or dial 611)

When the Code Itself Is the Problem

Before spending hours troubleshooting, double-check that the code you’re entering is correct and supported by your carrier. Not every USSD code works on every network. Codes are carrier-specific — a balance check code for T-Mobile won’t work on Verizon, and vice versa. Also, some codes that worked years ago have been discontinued as carriers update their systems.

If you found the code on a random website or forum, verify it against your carrier’s official support page before assuming your phone is broken. A quick search for “[your carrier] USSD codes” or “[your carrier] MMI codes” will give you the current list.