While traveling outside the UK, your cell number stays the same for anyone who wishes to call you. So people from the US should dial 011 44 7xx xxx xxxx whether you are in the UK or the US. The call is routed through the UK and then back to the US. It’s how international roaming works.
In the same way, your UK compatriots can reach you by dialing just 07xx xxx xxxx – the way they dial when you are in the UK.