If your iPhone 17 shows a "Not Registered on Network" error or displays an invalid IMEI, you won't be able to make calls, send texts, or use mobile data. The IMEI is a unique 15-digit identifier that lets your carrier recognize your device. When this number becomes corrupted or unreadable, your phone can't connect properly to cell towers.
You can check your IMEI by opening the Phone app and entering *#06#. If you see "Invalid" or a string of zeros instead of a proper 15-digit number, your device needs troubleshooting. Try these steps in order, as many network errors can be fixed without advanced IMEI repair.
Restart Your iPhone 17
Always start with a simple restart. This clears temporary software glitches and forces your phone to go through a fresh network registration process when it powers back on. To restart, go to Settings > General > Shut Down, or use the force restart method: quickly press and release the Volume Up button, quickly press and release the Volume Down button, then press and hold the Side button until you see the Apple logo.
Toggle Airplane Mode
Switching Airplane Mode on and off resets your cellular radio. Open Control Center and tap the airplane icon. Wait about 15 seconds, then tap it again to turn Airplane Mode off. This forces your phone to reconnect to your carrier's network from scratch, which can resolve temporary registration hiccups.
Reinsert Your SIM Card
A SIM card that isn't seated properly can cause network errors. Turn off your iPhone 17. Use a SIM eject tool or a paperclip to gently open the SIM tray on the device's frame. Carefully remove the SIM card, inspect it for damage, and blow any dust out of the tray. Reinsert the SIM and push the tray back in firmly. Power your phone back on and check for a connection.
Verify Your Carrier Account Status
Sometimes the issue is with your service, not your phone. An unpaid bill, expired prepaid plan, or account suspension will prevent network registration. Use another phone to contact your carrier's customer service or check your account status online through their website or app. Confirm your line is active and in good standing.
Reset Network Settings on Your iPhone
This step clears all saved network configurations, including Wi-Fi passwords, Bluetooth connections, and cellular settings. It can fix corrupted data blocking network registration. Go to Settings > General > Transfer or Reset iPhone > Reset. Tap Reset Network Settings. Your phone will restart. You'll need to rejoin Wi-Fi networks afterward.
Update iOS Software
Apple releases iOS updates that often include fixes for connectivity bugs. An outdated version of iOS 26 could have a known issue that's already been patched. Connect to Wi-Fi, then go to Settings > General > Software Update. Download and install any available update. After your iPhone 17 restarts, test your cellular connection again.
Try a Different SIM Card
Your SIM card itself could be faulty. Borrow a known-working SIM card from a friend or family member (or use an old one you have) and insert it into your phone. If the new SIM connects to the network without issue, your original SIM is likely damaged and you need a replacement from your carrier. If the borrowed SIM also fails, the problem is with your phone.
Erase All Content and Settings
This is a last resort. A full factory reset returns your iPhone to its original software state, which can eliminate deep-seated software corruption. First, ensure you have a complete backup via iCloud or your computer. Then, go to Settings > General > Transfer or Reset iPhone > Erase All Content and Settings. After the reset, set up your iPhone 17 as new and check for the network error. If the problem remains after a clean install, there is likely a hardware issue with the cellular modem, and you should contact Apple Support.