Q: my 3g wont work on my iphone 4 using ios7.1

Hello Alan,

It sounds like you are not able to connect to the 3G network on your iPhone.  I recommend the following steps when you are unable to join 3G/4G/LTE network on your iPhone:

Follow the instructions below for assistance with troubleshooting cellular data issues on your iPhone. After performing each step, please test to see if the issue is resolved.

1. Toggle airplane mode: Tap Settings, turn airplane mode on, wait three seconds, and then turn off again.
2. Restart your iPhone.
3. Ensure that your software is up to date:
   • Check for a Carrier Settings Update. Tap Settings > General >About.
   • Check for an iOS Software Update. Tap Settings > General > Software Update.
     Note: Some updates may require a Wi-Fi connection.
4. Remove the SIM Card and reinsert it. Allow the iPhone to acquire the network again.
5. If your SIM card has SIM PIN enabled, try toggling it off: TapSettings > Phone > SIM PIN.
6. Try another location. If a different location works, but the original location still does not, contact your carrierto report the issue.
7. Reset network settings: Tap Settings > General > Reset > Reset Network Settings.
8. Restore the iPhone as new.
9. Contact your carrier to:
   • Verify that the iPhone is properly set up on the account with the appropriate, current data plan.
   • Verify that there are no account-related blocks.
   • Find out if there are specific error messages in the carrier logs that could help determine why the issue is occurring.

If none of the above steps resolves the issue, contact your carrier, make an appointment at an Apple Retail Store, or contact AppleCare to troubleshoot further.

iPhone: Troubleshooting a cellular data connection

http://support.apple.com/kb/TS3780

Thank you for using Apple Support Communities.

Best,

Sheila M