There are actually two sites that offer this service on a pay as you go sim card. They are;http://www.sim4travel.co.uk
You place the sim card in your unlocked mobile phone and in most countries you then start to receive calls to the number provided for FREE. The former one uses a Liechtenstein number but using the excellent http://www.yourcallworld.com
Liechtenstein is one of the countries they offer free calls to. Making calls is cheaper too and I advise you to visit their websites to check out the charges.
I'm not linked to any of these sites, I just travel a lot and am looking for cheap ways to let my kids and friends phone me. I also got a little fed up with huge phone bills from calls from work on my mobile while away. This saves me that problem.
Please check that your mobile phone provider is listed as offering free calls on the yourcallworld.com access numbers.
I hope this has been of help to someone.
The sim cards can cost up to 30 pounds each, sim4travel include 10 pounds of credit for about 29.95. It might seem like a lot of money to pay for a sim card but those that have had even one mobile phone bill after a trip abroad know just how much it can cost and this is definately worth it.