Telephone charges for calls within the USA are rather inexpensive. Calls from residential or commercial landline phones can be as low as two cents a minute. On cell phones, the prices are a bit higher, but still rather reasonable at 10 cents a minute (or a bit more) during business hours and much lower on evenings and weekends. The same rates typically apply to calls anywhere within the USA.
If you use your international telephone within the USA, you may be charged some exhorbitantly high rates. You should check with your local telephone service provider to see what those rates might be. I suppose you could buy a SIM card to use a local USA telephone carrier. If you are using a GSM only telephone, you must connect to the T-Mobile or the ATT networks as they are the only ones that currently provide GSM coverage in the USA.
I would explore the possibility of purchasing a few disposable telephones for usage in the USA. Prepay mobile telephones are now available at discount stores, drug stores and other retail establishments. I have seen them as low as $29. You can easily purchase one for less than $50. You must prepurchase the minutes before usage, but the per minute charges for calls within the USA are typically only 10 or 20 cents a minute.