I am on PAYG, but actually don't use it often at all - this was a call that didn't even go through, only answering machine and I hung up immediately.
What's frustrating is that there's always some clause usually hidden deep within a contract that just enables them to efficiently rob you of all credit.
Oh, and this company at least seems to have time limits to their 'deals', usually of a month, if I don't use it within that time...