If you have a sim card and do not use it but would like to keep it active then I suggest you insert it into a mobile as if you dont use your number for a period of time then you number becomes inactive and gets sold again like a new sim card. It happened to me last week, when I found an old sim card, I put it into a mobile and called from another mobile instead of ringing on my phone it rang somewhere else and someone else picked up. Luckily I didnt need the sim card it was just spare.
Yes that's true I have been through the same thing. I had a spare sim card which I did not use and when I thought of using it, it wouldn't work. I dialled the number as I had it and to my surprise I found out that the number now belongs to someone else.