Have you recently updated or downgraded your iPhone firmware, or jailbroke/unlocked your iPhone to use it with a different carrier and got the following error:

Could Not Activate Cellular Data Network: You Are Not Subscribed to a Cellular Data Service?

This error often appears after jailbreaking and/or unlocking and it is mostly happens because your iPhone can’t connect to the internet using the cellular data network, and you can’t send SMS or MMS…
NOTE: Your iPhone will still work for everyday regular “phone” use but the data part is missing… Annoying isn’t it? Especially if you’re paying for it and not getting it.

The first thing to do in that case is to reset the network settings and reboot your iPhone. If it still doesn’t work, then there is a more radical fix that will put everything back on track.

First, get on wifi and visit this website http://www.unlockit.co.nz/#_Custom from your iPhone. You will see the welcome page with a CONTINUE button at the bottom… Press the continue button… You will see “Disable Data (FakeAPN)” and “Custom APN” buttons… + Instructions.

If you press the “Disable Data (FakeAPN)” you will see “Install Profile” page where you will be able to install this Fake APN… However… This is not the solution for the problem we’re having here… (I’m just trying to show you what FakeAPN does)… So to fix our problem we have to create a CUSTOM APN…

Here comes the solution for your problem… Click on “Custom APN” button… You should see a “Custom APN” window with some fields (which will populate automatically), V3 Tethering + MMS Field (Switch it to Yes if you want) and the “Carrier” drop down bar…

Under “Carrier”, tap the drop down menu and choose your carrier. Note that there are many carriers listed there and that it might take you a bit of scrolling down to get you where you want. So choose the right carrier and tap “Done”. This should populate the custom APN info.

Once the custom APN fields are populated, tap “Create Profile” and wait a bit. It will exit Safari, and take you to the settings application. Finally, just tap “Install” and this will set up your custom APN on your iPhone. Reboot your iPhone and you should now have Edge and 3G connectivity.

That’s all there is to it… Just reboot your iPhone and you should be back on line…