The inconvenience of this is partly eliminated by using a Personal Number. Don't automatically assume that a Sim Only is the best value for you, however. You hang up and the switchboard in the USA rings you back on your mobile. You answer the call, and have a US dialling tone. You pay for both legs of the call, but the charges are fairly low.
International calls are also competitive.
I haven't tried this system myself, but plenty of people find it useful. I'd be interested to hear what people think of the service. It does seem a lot of messing about. If you have an Orange or One2one phone, there is an easier and cheaper alternative, though. Personally, I use AXS and OneTel .These also offer 0800 number access and allow you to make these cheaper calls from any number, not just your home or office number.