Might be worth a mention, giffgaff to giffgaff calls are FOC (free of charge) this also includes if you haven't topped up credit upto 3 months.
Also they are changing their terms next month, a £10 goody gets 250 UK Minutes, Unlimited UK texts, 1GB Mobile Internet (Tethering will be allowed)
You will have to pay £12 to get 250 UK Minutes, Unlimited UK texts, Unlimited Mobile Internet (Tethering will not be allowed)