Which mobile plan is cheapest?

I have used Telstra, Optus, Three but none of them as cheap as TPG:
http://www.tpg.com.au/mobile/plans.html

Just spend 13$ or 20$ a month and get huge credit 150 and 300 respectively.
Why paying expensive plans with Optus and Telstra while TPG is extremely cheaper?

The only reason for using three is that its plan is capable for free international calls and have free mobile phone

Make free international call from mobile phone

I used to use 10$ top up optus to make 200 minutes international calls to Vietnam. However, it is really cost when I need chat with my family and friends.

After an ages, I found a solution to make free call. If you have a lot credit with your cap. Just call to 0401863814, and enter your international phone number. As you call to an Australian mobile number, it doesn't cost international rates but national rates only.

It does save money.

Details at website below:
http://www.smartcalling.com.au/index.html