The most common fix is to verify and correct your phone's SMS Centre Number (SMSC), which handles outgoing messages – you'll need to get the correct number from AGL. Also, try restarting your phone, checking your AGL plan for SMS allowance or balance issues, and resetting your phone's network settings. If these steps don't work, contact AGL customer support immediately; they can check your account for any blocks or provision your service correctly, and if unresolved, you should definitely pursue canceling the plan.
