MTN cellphone contract gone wrong

My dad has an cellphone contract with MTN for 2 cellphones totalling R350 a month however his being incorrectly billed 1st they wanted +R2000 then R5200 was deducted from his account recently he received another statement saying they want another 5k he had been to the MTN branch and all his told is that they will assist him after a month but how can that be.