Posted a few days ago, but TopCashBack have increased their cashback value to make this deal even better.
Unlimited Texts / Unlimited Calls / 20Gb Data
12months @ £20 = £240 (BT customers)
£90 Amazon voucher
£90 TopCashBack cashback
So altogether, £240 - £90 - £90 = £60, or £5 per month.
Hurry though, the BT deal ends at midnight tonight.