Accounts not getting billed for monthly DID charge

Hello, we are running ASTPP 3.0 on Centos with the WHMCS integration module purchased from Inextrix. We have just discovered that our customers are NOT being billed monthly for the DID's that they have. The accounts are in prepaid mode, and so we don't expect to see invoices, however, doing calculations, we have confirmed no DID monthly charges are happening.