Why can Vodaphone charge $5 for international day pass, and Telstra $10?

[Kimee]

Just wanted to know why Telstra charges $10 for international day pass that Vodaphone charges $5 for?
I am going to the US on Tuesday. My travel companion is with Vodaphone and only has to pay $5 a day for the same (calls/data/country). I have to pay $10 a day because I am with Telstra. I don't understand why Telstra is double the price. Can someone please explain this to me?

[Jupiter]

Vodafone have subsidiaries all around the world. They can use their own backbone to transmit data and calls at a greatly reduced cost.