How to activate the SMS channel on IFTTT on the Optus network in Australia.
Assuming your mobile number was 0410 123 456.
Drop the leading zero = 410 123 456.
Add the Australian country code 61 to the front = 61 410 123 456.
Add 00 to the front of the number 00 61 410 123 456.
Remove all the spaces = 0061410123456.
Use this number (0061410123456) to active the SMS channel on IFTTT.
Hat tip to @Trail_929 for the pointer.
If this works for you on other networks in Australia, please let me know.