Disappointed client

We bought a phone from Dunn's in Tzaneen via PayJoy contract in 2022 December, we finished paying that account in December 2023. On the 05th of February 2024 they blocked the phone and claiming there is an outstanding amount, yet they are confirmed that the phone is fully paid and there is nothing else we need to pay. We sent an email asking for this error to be corrected but there is no response. I honestly don't know what to do now.