To re-enable your carrier MMS setting, you need to enable Carrier Bundle Support on iTunes
1. Close your iTunes and go to command prompt
2. Navigate to you iTune installation folder, normally, it will be "C:\Program Files\iTunes\
3. Type iTunes.exe" /setPrefInt carrier-testing 1 and press enter
4. Restart iTune
Now, download your carrier ipcc file. http://www.mediafire.com/IPCCfilesFORm1SH provide good ipcc file for Singapore Carrier
Do the following to restore your carrier ipcc
1. Reset your network setting at Settings/General/Reset/Reset Network Settings
2. Plug in your iPhone and start up your iTunes.
3. Choose your iphone under “Devices”
4. Go to the “Summary” tab.
5. Hold “Shift” on your keyboard then click on “Restore”.
6. Under Pop-up Window, Change File Type to “Carrier configuration(*.ipcc)” and browse to your directory where you stored your downloaded ipcc file and select it.
7. Go to Settings/General/Network/ and see if Cellular Data Network appears
8. Power off and restart your iPhone
9. Test send and receive MMS from your iPhone. By default, you do not need to change any setting.