Australia 1800 numbers
The following answer assumes you are in Australia and using an Australian mobile phone account
Most people join a teleconference by dialling a 1800 number.
Calls to 1800 numbers are free from mobile phones, so no charges will appear on your telephone bill.
Dialling a regular number
There are no local calls when using a mobile phone.
Calling to the person next to you costs the same as a call to the other end of the country so if you are not using the 1800 number, there is no reason to feel you need a local number.