Im on T-mobile and for £35 they give me £180 credit a month to use on anything (apart from 08 numbers etc, the ones that cost like £3 a min) and just to add a cherry to the top of the bun they chuck in unlimited evening and weekend calls
I have only ever got close to using the £180 once back when I had a long distance relationship and it had problems
The only drawback to mine is it was an 18 month contract, the 30 day one seems like a good idea, especially for younger people.