En play.com me piden el "Issue number" ¿que es eso?

Yo y un colega intentamos comprar un juego en Play.Com, y en los datos de la tarjeta nos pidieron el "Issue Number", pero como no sabiamos lo que era, intentamos seguir con el pedido. DEspues de haber enviado las fotocopias que pedian y haber recibido unos mails diciendo que lo habian aceptado todo y que todo hiba bien, recibimos este mail:

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"Dear Customer

Thank you for your order with Play.com.

When we tried to process your credit/debit card it was declined by your bank. We
would ask you to please check all of the card details carefully.
Your CREDIT CARD may have been declined because the card number, expiry date or
start date is incorrect.
If you are using a DEBIT CARD the card number, expiry date or start date may be
incorrect. FOR Solo and Switch cards the issue number should also be checked.
If you have made any changes to your account details and have clicked the 'Save
changes' button we will automatically try to process your order again.
If you have checked your details and they are correct it may be that your bank
has declined your card as part of a random security check. In this situation
you will need to contact your bank to allow the card to be processed.
So you don't miss out and to get your order to you as soon as possible, we have
reserved the item you have ordered, and will dispatch it as soon as we can
process your card.
Kind regards

Play.com"
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No se mucho de Ingles, pero por la pinta piden los del "Issue Number" ese. En definitiva, que me gustaria saber que narices les pongo.

Gracias


EDIT: los siento, acabo de ver que hay un hilo de dudas de play.com, ahora voy a poner este post ahi, sorry.