As the system is global and sending SMS to the global number needs to have the country code, so, you'll need to use the +1 in the case of using the USA number. It actually not possible to set +1 by default as this can be different to the other people.
Suppose, our country code it +880, so, if it is set to +1 by default, it won't be reliable for us.
However, I'm adding a feature request on our log to select the country first and then it will include the country code automatically. We'll consider this in the future 🙂