GSM go with straighttalk.
$45 for a month. unlimited voice.
http://www.straighttalk.com/ServicePlans
Are you bringing your own phone and only need a sim card? International phones work on AT&T's network so you could maybe go to an AT&T store and get a no-contract sim card from them.
EDIT: Actually, if all you care about is voice, I think T-Mobile would work too. They have a pretty good prepaid outfit that should fit your needs and it's cheap.
I am bringing my own phone and have a sim car. Looks like my best option is tmobile. The only thing is my one hour meetings.
There are tons of options. How many minutes do you need for this one month?