An “invalid MMI code” or “connection problem” message after entering a special dialer code does not automatically mean your normal calls or texts are broken. Some codes run on the phone; carrier or USSD codes request a network service. The code may be unsupported, mistyped, sent through the wrong SIM, or unable to reach your carrier. Start with the exact code and a normal-service check—do not try random “secret codes” or change calling features just to clear the message.
First, identify what failed
- Write down the exact code and what you meant to do. Note your phone model, carrier, country, and selected SIM or eSIM line. Keep your phone number and IMEI private.
- Make a normal call to a trusted number. Check whether an ordinary text and mobile data work too. A failure limited to one special code differs from a general loss of service.
- Confirm the code in current instructions for your exact carrier, service, and region, or in your phone maker’s guidance if it is a handset diagnostic code. A code from another carrier or service may not work—and some codes change where calls go.
If normal calling, texting, and data work
Re-enter only the verified code exactly as documented, choosing the intended active line if the phone asks. If it is a carrier feature, ask your carrier whether that feature is available and provisioned on your line. A single failed code does not prove you need to reset network settings.
If you only need your IMEI, you may not need a dialer code. On a Pixel or Galaxy, open Settings > About phone. Samsung notes that its IMEI dialer shortcut works only when supported by the carrier. Share an IMEI only with a trusted service that needs it.
For a Galaxy hardware test, use Samsung’s documented Samsung Members > Diagnostics > Phone diagnostics route instead of experimenting with forum-listed service-menu strings. Available tests and menus vary by device and software. See Samsung’s diagnostic instructions.
If ordinary mobile service also fails
Check whether the intended SIM or eSIM line is active, airplane mode is off, and there is usable mobile coverage. On a Pixel, Settings > Network & internet > SIMs shows whether Use SIM is enabled. Try a brief airplane-mode cycle or restart. If you see a SIM error, recently switched carriers, or suspect an outage or inactive account, contact your carrier. Google and Samsung both distinguish line, network, account, and phone causes.
Consider a network-settings reset only after following your phone maker’s instructions for a broader connectivity fault and understanding what it removes. It is not the first fix for one failed code. If you urgently need to reach someone and normal calling is unavailable, use another working phone or communication method.
Fixes to avoid without a specific reason
- Adding a comma or plus sign: these change the dial string; neither is a verified universal fix.
- Trying lists of carrier codes: a code may be unsupported or may alter a calling feature.
- Changing radio menus, VoLTE, or security settings: these are not established general remedies and may impair service or protection. Do not disable Galaxy Auto Blocker merely to try a forum-listed code.
- Removing the SIM or resetting all networks immediately: disproportionate when normal service works and only one code fails.
What to tell support
Give your carrier the exact code and error, phone model, selected line, country, approximate time, and whether normal calls work. Ask: “Is this feature supported and provisioned for my line, and what is your current documented way to use it?”
Sources
Google Pixel mobile-connectivity guidance; Samsung calling troubleshooting; Google Pixel IMEI instructions; Samsung IMEI instructions; and Samsung Members diagnostics. Carrier code syntax is service-specific: compare AT&T wireless call forwarding with AT&T digital-home-phone star codes. These are examples, not universal dialer instructions.
Related: How to Extend Ring Time on Android Before Voicemail.
Matthew Burleigh has been writing tech tutorials since 2008. His writing has appeared on dozens of different websites and been read hundreds of millions of times.
After receiving his Bachelor’s and Master’s degrees in Computer Science he spent several years working in IT management for small businesses. However, he now works full time writing content online and creating websites.
His main writing topics include iPhones, Microsoft Office, Google Apps, Android, and Photoshop, but he has also written about many other tech topics as well.