What are the chances that someone is somehow using a clone of my cellphone number or account?

I've been occasionally been receiving calls for years now from people trying to reach a (porn!) video rental store. Yesterday, because the caller was particularly chatty, I discovered the caller had MY telephone number written down or printed (not certain which) and had not simply mis-dialled. We cross checked the number and it was mine for certain he had there. He steadfastly insisted he had used the very same number to order the videos in the first place (that he was now calling to complain about). In fact, he was so sure that he insisted I was a part of a ruse to stitch him up with bad DVDs!

I pay little (almost no) attention to the telephone bill, it's always at (or slightly above) the normal monthly contract rate and I merely pay it without scrutiny. I wouldn't notice the odd additional calls granted, but I can say with a fair degree of certainty that nobody is milking my cellphone account.

I plan to do some searching on the 'net to try and get some background information on what hacks/exploits are available on cellphones before I speak with my network provider (Orange).

If anyone here can confirm or deny whether any of this is even realistically possible (as opposed to being hypothetical nonsense) I'd be very grateful.