Optus don't support video calling over their network domestically. You'd need to use a 3rd party app like Facetime or Skype which would in turn use your data rather be a standard cost like making a call
@Usrnam, you can find all rates in your Plan CIS.
All for use in Australia
Standard National Video Calling: 20c per min (deducted from your MyCredit balance)
When using your phone within Australia
Standard international video calling $3.60 per min (deducted from your MyCredit balance)
In saying that, as @SamSam has mentioned, we don't offer support for Video calling on the Optus Network. I'm really not sure why it's still referred to the in CIS.