How do you message a US mobile number from Australia?

The country code for Australia is +61. Additionally, you must omit the trunk prefix 0 from the Australian domestic number when dialing in international format, so mobile numbers begin with +61 4 instead of 04. On a GSM mobile, you should enter the number in full international format, beginning with the plus sign. On a non-GSM mobile, substitute your international access prefix (e.g., 00) for the plus sign.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, and many other countries use different prefixes.)