I've used budgetcom and alpha telecom.
Budgetcom is great if you're at home. Calls work out at 1 penny a minute + a 5p connection charge. As it's an 0845 number it's not idea to use it from a payphone as you'll pay about 10p a minute, or a British cellphone where you'll pay about the same.
Alpha telecom has an 0800 number which is great for payphones as (as I recall) still allow free 0800 calls and cellphones if you have a plan with lots of free minutes through it 020 number. The lines are always crystal clear and there is a decent customer service line. With their pre-paid US card it works out at about 3 pence a minute and for the line clarity it's an extra couple of pennie well spent.