After upgrading my iPhone from Jailbroken 3.1.3 to iOS4, I couldn't check my voicemail and I had no internet (3G or edge).
Im a PC so this instructions are for PC users.
Forget about calling AT&T, they wont help you at all.
PLEASE READ ALL THE INSTRUCTIONS BEFORE YOU DO ANYTHING, AND BE AWARE OF WHAT YOU ARE DOING!!!
First thing you want to do is BACKUP ALL YOUR CONTACTS, APPS, RINGTONES, NOTES, PHOTOS, EVERYTHING!
- Put your phone into DFU mode.
Connect your phone, open itunes, let it backup, then HOLD THE POWER BUTTON AND HOME BUTTON. When you hear the first connect sound, release the power button. When you hear the second connect sound, release the home button.
- iTunes will ask you if you want to restore your iPhone to factory settings. Click Yes.
- After a while it will ask you if you want to restore from your backup. Unplug your phone, check your voicemail. It will call your voicemail, then hang up. Call your voicemail again, and it will ask you to set a password. Set a password, hang up, and visual voicemail should work again.
- Reconnect your iphone to itunes, and set up as new iphone. Let it sync everything from your outlook and itunes (contacts and apps), then sync your music, videos, ringtones, etc. etc.
-Thats it. You should have voicemail and 3g/edge
I tried to restore my phone from my backup, but it messed up the voicemail 3g/edge again.
Feel free to post other fixes you find in the comment area.