Over the summer I went to america and used Rebtel's cheap abroad calling to contact my family. Calls cost less than a penny per min.
As a new customer just create an account and add £7 credit to the account Rebtel will double it and quidco will give you £18 cashback.
Mine has settled into my bank today (about 2 months after purchase) so i can confirm they do pay.
Thanks to "SaeedB" thats now £14 credit for £7 and in a couple of months £18 back so overall your £11 up and have £14 worth of credit to use on calls and texts.
Its actually £8.03 to top up £7 call credit. They will charge you £1.03 for VAT.