They'd be considered unpaid, and if your company had creditors at liquidation then you would actually be called on to pay up to that amount to them. It's what "limited" liability means - it's not that shareholders have no liability to creditors at all, it's that their liability is limited to the amount of share capital.
By the time shares are publicly traded, the capital will invariably be paid up so there's no need to go back to shareholders in the event of liquidation/bankruptcy, but for private companies it's perfectly possible for it to remain unpaid.
In practice you can just reduce the nominal value of the shares enough to make the amount to be paid relatively trivial. It doesn't stop you later injecting more money on top of that in various ways.