If its a pay as you go, then over a certain amount of time with no use they do seem to cancel that number.
Its unfair, however with the amount of numbers that are around these days i guess its only going to be a matter of time before we run out, so re-using old inactive numbers seems logical.
You can get a free SIM card from the 02 site and you get 500 free txt messages on it as you top it up by �10 every month.
I just top mine up every 2 months, 1 month on free txt then the next i cut into my balance so its very cheap.