Interswitch Airtime Recharge API
Virtual top-up (direct airtime and data) and e-pin voucher delivery for MTN, Airtel, Glo, and 9mobile. Same biller/category/payment-item flow as Bills Payment, with category ID 4 reserved for airtime billers; phone number is supplied as `customer_id` on the `/api/v2/quickteller/payments/advices` payload.