If you are already using the correct SMS format and the messages are still not going through, please go to your Contacts list and edit the contact manually, changing the number to the full international format by including a +, international code, state code and dropping the first 0.
Example: Instead of having the contact saved in the national format you should change it to the international format.
This is a standard format around the world and you will not be required to change it again when you return home. Some smartphones will recognise the number you are trying to text and they will reformat the number and send the message from your contacts automatically.
Note: Some smartphones also keep a record of text messages in one single conversation. This means that because you previously messaged this number in local format, your smartphone will continue to revert the phone number into a local format which blocks you from sending international SMS. Note that this issue is common among iPhones and you must follow the steps:
Step 1. Select the person you are messaging and delete the SMS history
Step 2. Make sure they are not saved in your contacts in the local format
Step 3. Make sure they are not saved in your favourite contacts
Step 4. Please ensure that the local format phone number is removed from any part that your smartphone may be picking up the number from
Step 5. Please retry sending the SMS. If the issue persists please try deleting this contact altogether and then re-adding them to your contact list in international format.