DON'T BUY ONE
I know; you want one. You can think of loads of things that you could use it for. Imagine, all of your sim cards on one. No more carrying 2 or 3 mobiles around.
All I can say is if you live in the UK and have a UK mobile with a UK sim card don't waste your money because they cannot be cloned. At least not with the items available on ebay for under a tenner!!
It all comes down to breaking the encryption algorithm on the sim card. The older version 1 cards as used in the 1990's and still used in many countries who do not have as advanced a system as we do here have had their algorithm cracked.
With the latest version 2 and in some cases version 3 cards that are in use here in the UK are very secure. With the sim cloning devices available on ebay you are likely to incur the expense of buying a new sim card as well as wasting your orginal money.
The programs used rely on breaking into the sim card by brute force - ie. trying every possible code to gain entry until the right one is found (I have read that you stand more chance of winning the lottery jackpot two weeks running)
The problem arises because the sim card producers build in a counter that checks every attempt and when you exceed a set limit the card stops working. FULL STOP.
You may be thinking that the information I have dug up on the net is out of date and that the guys selling these things, usually from China (despite pretending to be in Manchester) where they do still use the older 'cracked' sim card system, have found a way into the new ones. Well, my guide is not just based on anecdotal evidence. As part of my long quest for a refund after buying and frying (buying the sim card cloner - frying 2 sim cards; the second after being sent a link to 'updated' software by the seller!!) I checked through his last months feedback and found TWELVE other purchasers of the same item who had left negative feedback stating that the item didn't work. Bear in mind that some people may not bother to leave negative feedback as they fear a tit-for-tat negative feedback in return and others just can't be bothered over a sub £7 purchase.
The following quote comes from the much respected expansys website: "There are ways of cloning pre-2002 SIM cards. If yours is newer than that, there is currently no way of cloning it."
YOU HAVE BEEN WARNED - THEY DON'T WORK - DON'T WASTE YOUR MONEY