score:5
There are many available options as far as prepaid services available in the US:
Most of these sell GSM phones that are enabled for US and some are even able to be used in Europe but you will need to have a tri-band or quad-band to be able to do that. Now as far as IS-95 commonly known as CDMA is that outside the US/Canada the coverage is not as common and not as available.
EDIT
Upvote:5
How about 1700/2100?
Since you have no need of data service...
T-Mobile USA has a pay as you go prepaid plan which, when you add a $100 top-up card, switches your prepaid account to a 1 year expiration, which remains in effect for the lifetime of the account, and only requires adding $10 per year before it expires in order to keep the account active. (Their website doesn't make this clear, but that is how it works. I've had one of these since 2008.)
You can purchase a device from T-Mobile or bring your own device (see eBay US for ideas) and get a SIM for $10. You can buy the device or SIM online or at any T-Mobile shop. Top-up cards are available at many grocery stores and pharmacists around the country, or online.
Voice coverage is pretty good throughout the US, excluding some rural areas in the west where nobody lives anyway. And yes, they have coverage in Mammoth Lakes.