Both should allow you to receive calls in US from Aust no problem (the caller in Aust is paying to call you, and you just need local call tiime to receive), but making outbound calls from US number is where costs do apply. Also, side point, AT&T give unlimited international SMS in all their p...

Hi MissBasset, AT&T charges calls to Aust mobiles at about $1.09/min, and that requires credit above what you pay for the plan. They also have a $5=250min international call pack, but that only covers landlines in Aust ... but to be honest, obviously depends on preference and cost sensitivity...

As someone who deals in local and roaming SIM cards all day (I run a SIM retailer based in Melbourne), I'd add that T-Mobile is probably not known to have the best speed/coverage outside a few of the biggest cities. I do sell both AT&T and T-Mobile, but ATT has far superior coverage and speed...