What kind of cell phone do you have now? A smart phone? Flip phone?
If it is a flip phone, the only way you'll be able to make calls with it is to use credit from the Spanish SIM you have in it - presumably at expensive roaming rates. You might not get many minutes out of it, though if you have enough credit that you won't use up anyway, it may work out. Add a +1 to the front of US phone numbers to call with it. So to call 212-555-1212 dial +1212-555-1212 .
(To get a + on a cell phone, hold down the 0 (zero).)
If you're using a smart phone (Android?), and you have signed into your Google account when setting up the phone, you should be able to use Google Hangouts to make free voice calls from the phone while you have a WiFi (at the airport?). Install the Google Hangouts and Hangouts Dialer apps.
Similarly, you can try a free app called "TextNow" which will let you make free calls to US phone numbers from a smart phone, as long as you have a WiFi connection.
Last resort: get some change and find a pay phone. They do still exist.