How do you clone a SIM card?

[up4it]

How easy is it to clone a SIM card? And do you need the original SIM card to be able to do this?

Basically I have just found out that 26th July to 31 July someone has been making calls/texts which have been charged to me for the amount of £152 (I have not used this mobile phone for 4 months) and for August I have been charged with £745

I spoke to my service provider and they insist that I must have given my SIM to someone in order for them to do this, I checked last night and my SIM card is still safely at home (not even in the phone).

It seems to me that it is just going to come down to my word against theirs i.e. they say I have given my SIM out I say that I havent

I would like to understand how you can clone a SIM or charge calls against my account so that I know what I am talking about when I speak to my provider again about this

Can anyone advise?