Calls in arrears means that the company has demanded money but the shareholders have not yet paid the money. Calls in arrears is an asset of the company and the company has a right on the same. 

We show the Calls-in-Arrears Account under the Main head share holder's fund and in the Notes to Accounts on Share Capital to the balance sheet as a deduction from the amount of ‘Subscribed but not fully paid-up’ under ‘Subscribed Capital’.