General principles of Calls on contributories:

If it is necessary to make calls on contributories the liquidator draws up a list "A" of contributories who were members at the commencement of the winding up and a list "B" of contributories who were members within the year preceding the commencement of winding up.  A list B contributory has liability limited by the following principles:

(a) he is only liable to pay what is due on the shares which he previously held and only so much of the amount due on those shares as the present holder (a list A contributory) is unable to pay;

(b) he can only be required to contribute (within the limits stated in (a) above) in order to pay those debts of the company incurred before he ceased to be a member which are still owing.

As stated above a past member ceases to be liable altogether (on partly paid shares) if the company continues (without going into liquidation) for a year.