BT shelves them for around 5yrs after a business ceases trading. After this period, its put back up for re-use and often given out to a new business.
We were given a new number in February last year, all was well until April. We were then getting around 7 calls a week requesting wedding packs and room availability, our biz is sats and aerials. And this number had not been in use for 5 years!!