Let's assume your USA mobile number is 310-123-4567.


For someone in Australia to call you from their landline, they will dial 001113101234567.  That's 0011 (the exit code from Australia), 1 (for the USA) then the full USA mobile number.


For someone in Australia to call you from their mobile, they will dial +13101234567 (yes, they must enter the + symbol).


For someone in Australia to send you an SMS, they will send the SMS to +13101234567 (yes, they must enter the + symbol).


It will cost the person in Australia however much their phone plan charges them to call or SMS to an American number (this varies from plan to plan).