The best deal I've found is the ASDA sim card, available in store free of charge when you put a minmimum of £5 credit on it. Calls cost only 8p per minute (any network) and texts 4p each. Beat that! You may have your existing number ported. You can top up by credit/debit card via your mobile phone or at a newsagent. I've been using it for several months trouble-free.