As you'll surely plan to call home eety is the very best choice for you.
As you can see
here (click) calls to the US are charged only € 0,12/min.
If your mates also get an eety SIM card, you will reach each other at € 0,09/min.
Btw incoming calls are NOT charged in Europe, as long as you stay in the country the SIM is from. So if you're being called in Austria - no matter from where - that won't cost you a Cent!
eety is available at e.g. Cosmos, Saturn and Austrian post offices.
In order to use eety you'll need a phone that supports GSM1800.