MTN refusing to refund my Money

I was overbilled on roaming charges in May 2024, contested and got the refund but as a credit, typical companies only take and if they owe they force to retain s credit on account. Cancelled contract and a refund of 4k in Feb/March and to date no refund. They keep asking for bank statement and POP for a debit order and further comprehensive record on transaction is in their database from July 2024 to March 2025!