I have since contacted my mobile phone provider to enable it. Do I need another confirmation message sent? 

No. If your SMS phone was already submitted to our database, you should be confirmed to receive SMS alerts. However, you can check if you are opted-in by texting the word “SUBSCRIBE” to 23177 or 63079 and you will receive the following reply:

HELLGATE: This phone number has already been opted in. More info text reply "HELP" or "STOP 7282402" to opt-out