Are you using an iPhone?
If you are using an iPhone, and text messages to Australian mobile numbers won't send / constantly fail, please note that there is a known bug on the Apple Support Forums concerning some iPhones and their ability to send an international format text message. What happens is this: the iPhone reverts back to sending the text in non-international format. To rectify this, please do the following:
1. Let's assume that you can't send text messages to a contact called "Art Vandelay". Every time you try to text Art, the texts fail.
2. Go into your Messages App and completely delete every single text to or from Art Vandelay. You must literally delete every single text including any group texts where Art Vandelay is a recipient.
3. Delete Art Vandelay from the Contacts App. We literally mean delete Art Vandelay completely from your Contacts App. Don't just delete his name or his number. Go to the Contact name, tap Edit in the top right, then scroll to the bottom and select Delete Contact - completely obliterate any reference to Art Vandelay in your Contacts (Obviously you should at this stage write down the phone number/s).
4. Once you have deleted every single text to and from Art Vandelay, and you have delete Art Vandelay entirely from your Contacts App, then compose a new text message to Art Vandelay, making sure to compose the text in international format. For example, 0404-123-456 changes to +61404123456.
We repeat: until you do literally all the steps above, text messages to Australia will not work.
Are you using any other phone?
If you can't send an SMS to an Australian mobile, please complete the following troubleshooting steps one by one, and then try sending an SMS after the completion of each step. Please do the steps in the order they appear below:
1. Turn your phone completely off and then back on.
2. Confirm that you are composing the SMS in international format. For example, to SMS 0404 123 456, you must send the SMS to +61404123456 (you need to actually enter the + symbol at the start). Do not add any other prefix to the mobile number eg don't add 011 or 0011 etc.
3. Confirm you are in fact sending an SMS to an Australian number. Please note that you can't SMS to an Australia Post Travel Sim or a Woolworth's Travel sim as they are neither USA or Australian mobile numbers.
4. Have the Australian mobile send you an SMS and then reply to it. Remember they must send to you in international format. So if your USA mobile number is 310-123-4567 they must SMS to +13101234567 (remember they must actually enter a plus symbol).
5. Please ensure that iMessage is activated if you are using an iPhone - go into Settings, Messages - if iMessage is on, then it it off and then back on. Or if iMessage is off, turn it on. Please also remember that iMessage will only work on 3G if you are on the USA Extreme Plan or USA Max Plan. You can use iMessage on the USA Saver, USA Daily and USA Budget Plan only when connected to wi-fi.
6. Consider using an alternative app to send SMS. For example download "Whatsapp" for 99c or "Viber" for free. When you first install the app it will automatically set-up all your contacts who also use this app. You can then send and receive SMS, pictures and video through the app, which uses data only.
TIP: To enter the + symbol, hold down on the 0 button key