From the UK, I’ve been using www.telediscount.com for a while now (it was recommended to me on FT) to call Japan. I pay the cost of a local call - period. No need to purchase a card. I’ve found that the following number works for most places I’d want to call: 0845 2442442 – I just enter the full number I’m calling (preceeded by 00) and finish off with a # to get connection.

Note:
The website will give you other access numbers to use – but I’m uncomfortable recommending any access numbers that begin with 09 – I have found that the prices advertised in Telediscount for places such as India don’t always match with the prices given for the same premium number in BT’s website.
You can cross check the price of calling any number at: http://www.serviceview.bt.com/list/homepage.htm

Bear in mind that mobile companies can charge what they like!