How to enable calls to the USA from the UK ?

Hello MichaelY_800484 . As JJP2 says, you will need to purchase an add-on.

Details of all costs are shown in the Price Guide, specifically p20-p22: https://cdn.smarty.co.uk/files/SMARTY-Price-Guide.pdf

Fortunately, all you do to is buy an "out of plan" add on, and then dial as you normally would.

Calls to both landlines and mobiles in the USA cost 19p per minute and texts cost 6p each.