I think it makes sense - you want to call a UK mobile from the US using a UK registered mobile.
The international access code from the US is 011 (not 001) so to call a UK landline from there to say Manchester is
011 44 161 xxx yyyy
To call a UK mobile is similar
011 44 7799 xxxyyy
It doesn't matter what the phone is, it is an international call from the US to the UK.
Try this site
http://www.countrycallingcodes.com/
Let me know if this doesn't answer your question.