On my Samsung Galaxy Ace, it says "unregistered SIM". I shut it down, took out the battery + SIM card, and still no luck. It says I have service, but am not able to send or receive texts. What do I do?
Try Settings > Wireless and networks > Mobile Networks (I can't recall what the exact screens are in gingerbread) and ensure that 2G is not enabled.
Try toggling airplane mode on and off.
Make sure your bills are up to date and you haven't hit your cap on SLP (if you have it).
If any of these don't solve it, it could be a faulty SIM card. Try your SIM out in another phone to see if it works and let us know the result. If it does, then you have a phone issue and should upgrade. If it doesn't, then try replacing the SIM card at any Koodo shop or retailer. $10 + tax for postpaid and $20 for prepaid.