$20 cash back from Telstra/Optus... what can I buy?

I got an offer from my NAB Amex a while ago stating that I can get $20 cash back from BOTH Telstra and Optus if I spend more than $20 before end of this month. I am not a Telstra and Optus customer and am just wondering if you guys can suggest anything good that I can buy from them… like iTunes card or gift cards?

I tried to look at their website but did not find anything interesting… any suggestions welcome.

Thanks!