CellC refuse to unblock a brand new phone wrongly blocked by them

My son has got a new Samsung Galaxy S20 Ultra from his insurance. This is a brand new phone. The phone is on the MTN network. The phone stopped working. After numerous calls and little help from MTN it was established that the pone was pbocked by CellC. Eventually CellC said that it was blocked due to non-payment. My son is not with CellC. CellC is unwilling to help and MTN is unwilling to help. Must he now throw a R300000 phone away. Surely MTN can contact CellCc and sort this out, they are unwilling to help