I googled your phone and it works on 1900Mhz, which is compatible with several US networks. My advice would be to get a pay as you go sim from Three. Forward all your calls to that number, then put the three sim in your phone. Now you can still receive calls as normal without having to give out a new number, but outgoing calls will come from the Three SIM card.
Three will allow you to use your phone in the US to make and receive calls to and from the UK for no charge (feel at home)
If you need to make a call to the US (or anywhere else) you can use a UK based calling card (Tesco for example). It’s ironic that it’s cheaper to bounce a call twice across the pond than to make a local call in the US, but everything goes through the internet nowadays anyway.