Are you on PAYG or pay-monthly? PAYG phones are auto-enabled for roaming once used in the UK first. Pay-monthly phones need to have roaming enabled by texting ROAMING to 150. Without this, they will not connect to foreign networks.
Call costs for roaming in the USA are at https://ee.co.uk/help/help-new/roaming-costs
If on pay-monthly, the Roam Further pass allows you to use your UK allowances in the US for £15/month
