Overseas callers can usually access 13 and 1300 numbers, depending upon the overseas network operator. Your customers can call your number by prefixing it as follows:
overseas access code + country code (61) + your phone word number
Example:
To call 1300-DIAL-ME from the United Kingdom:
overseas access code (00) + country code for Australia (61) + 1300-DIAL-ME
= 00 61 1300 3425 63
It is not usually possible for people abroad to call 1800 numbers through traditional telephone services. Some Voice over IP (VoIP) services, such as Skype, do support calls to 1800 numbers.