I have looked at their prices and they are selling those cards at a tremendous premium over what it costs to buy in that country. For instance, the Australian Optus card is $49 (US) and I believe I bought the same card for $30 (AUD). That is more than a 100% markup if Im doing my currency conversions correct. As a result the prices they advertise per minute seem based on the face value of a card, not what you are actually paying for the card and the time on it.

Likewise, the Virgin Mobile UK card is $40. You get about $6 of call credit. Calls to the US are 30 cents a minute. So for that card you can get about 20 minutes of calls to the US. If you calculate based on the price of that card that is about $2 per minute, which is twice what it costs you to roam with a US TMobile number in the US for instance.

Bottom line, it is much, much cheaper to buy a card locally or in some cases even to roam on a US account. As you add to the account the average price per munute comes down of course, but you simply can get the cards for far, far less in the country in which they are used.

Getting a Virgin Mobile SIM at Heathrow for instance is simple, several places sell them at the airport (The Link for one). Do a search on it, there are several threads about getting the cards locally as well as about which ones have better coverage and rates, in the UK and other countries. With Virgin SIMs you walk in pay the price (much, much less then Telestial) and put it in your phone and make a call. Easy as can be.