O2 offer some great SIM Only deals. There is no long term contract, no need to change your phone number and you can keep your existing phone. O2 offer a SIM Only with with 600 minutes and 1000 texts for only 20 pounds per month. You also get a choice of FREE Bolt-ons including UNLIMITED TEXT, UNLIMITED INTERNET, UNLIMITED O2 to O2 Calls, UNLIMITED Weekend or Landline Calls or 200 Extra Minutes.