Call cost confusion USA sim card

i am having difficulty understanding your call rates. there is a rate when using us number (+1), another for uk number (+44) and yet another for global (+?) number. for example when receiving calls the rate is 16p on us number but free on global number. is the global number the same as uk number?

in summary when is it free to receive calls/texts whilst in the us from any number?

please respond to maurice.jjj@tiscali.co.uk