Why would you even try? Surely "my card number doesn't work" is the simple and sane response. They don't need to know, or care, why.
Since we're talking about good support here, I have another story involving them:
When we started the contract with them 6Mb/s was the fastest available speed, a few years later it suddenly went to 70Mb/s for a week and then down again (which is fair, we had a 6Mb/s plan after all). I called them and asked if we could upgrade to a faster plan. They said they could now offer us a 50Mb/s plan and asked when I would be home. One hour later they sent a representative with a new contract, we signed, and the next day speeds went up to 50Mb/s again.
Tell them your card was lost - problem solved.