I use three, i only top up now and again. Your credit never expires. You put £10 on you get £10 worth of calls, ok the payg tariff i'm on also gives you 150mb of internet and 200 text free (I never use the internet, but the free text are handy). It also gives you 3000 free minutes to call people on the three network (3000 minutes over 45 days), which is great for me as most of my friends are on Three. Like i said the free stuff is exactly that, FREE, you don't have to use it, Put £10 on and its there untill its used, no fees or expire date.