Tyre refund not made as promised, nobody reachable

I rented a Fortuner in Namibia. On inspection of the vehicle, I found the tyres to be worn dangerously and asked for a replacement, which they did not have. Instead they sold me tyre and windscreen insurance. Thrifty policy is that you have to replace a tyre in case of a puncture at your cost, which they will then refund. That happened. I phoned them for clearance beforehand and supp**** them with the invoice when handing back the car. They promised a refund within 7 - 10 days. This is now 1 month later and no sign of a refund. To add insult to injury, I am cut off when everytime when transferred to somebody doing the refunds. And that after holding on for more than 5 minutes very time. Never Thrifty again!