MTN Not Yello...ing

MTN are the worse. A loyal customer for 25 years and some ********** activity on our contract and we are the baddies. One cannot speak to anyone at MTN other than *********** customer service staff. They allowed someone to purchase data on a contract totalling more than R16k and become the primary account holder - without any identification. We are expected to pay this. Please let me know how we can take this legal?