You want to call the U.S. from your mobile in the U.K.?
- have a look at Skype http://www.skype.com/en/rates
You can Skype from many devices now, not just a smartphone or PC.
If one or more both parties have smartphones then again look at Vonage Mobile, MagicApp, Skype, Google Hangouts, Viber or other VIOP app or even FaceTime (if both have Apple devices) for example. These will be cheap or even free if using a free or home wifi connection.
Another option is for you to swap out your SIM on each occasion to make an overseas call using a different provider, e.g. Lycamobile - pay as you go cheap international calls. Lycamobile goes over the partner O2 network (your Sainsbury one uses Vodafone, so check coverage first) but this could be an option if you don't have a smartphone.
Or are you going to the U.S. and want to call back home when you get there?
- again Skype?
I've used a travelsim.co.uk in the past (still got some credit on my old "holiday" SIM card), but my now my iPhone allows me to make virtually free calls from abroad back home now....using BT SmartTalk. If you are a BT broadband customer and have a smartphone (android or iPhone) you can go to a free wifi spot anywhere in the world and make a call back home for the same price as it would be from your home landline. Genius.