Public Switched Telephone Network (PSTN) has been in Australia since the 1930s, is a standard analogue telephone line that gives you one speech pair. If you want two people talking in your office at one time you need two PSTN lines. If three people are required to speak at once you need three PSTN lines. Each PSTN line has it's own telephone number, when advertising, if you require your lines can be in rotary. Rotary means you advertise one number and when it is in use it rotates to the next number in the telephone exchange. PSTN supports fax.

Integrated Services Digital Network, otherwise commonly known, as ISDN is a clearer and more dedicated digital line with the added feature of indial. Indial is a 100 telephone number range. One main number and 99 indial numbers, which can be allocated to individual extensions. Your service provider can bill Indial numbers individually. The indial range can be established on new ISDN connections only.

PSTN lines can be converted to ISDN lines with no ISDN facilities such as the indials. Unless moving to new premises there is no advantage to changing to ISDN.

Indial numbers can assist with marketing, by indicating the calls to advertised numbers. A computer software package will list exactly which telephone numbers were called, this can determine which advertising methods are working effectively.

Below is an example of how Indial numbers may be used to assist with marketing and used for a direct personal line for staff.