2 account numbers amalcomated

we where send a billing statement to the effect that we where owed an amount of R 16 000.00 they for 2 months deducted our service bills from account and then we received a lawyers letter to say we owed an amount of 30 000 where they calculated 2 amounts together how the got to that amount i dont know we made arrangements to pay the amount that bill came down to 10 thousand rand but now they still want me to pay the amount that they calculated together which is the amount that was wrong on one bil on a other account i stay at one property for11 years and there is 2 accounts on 1 property i have prepaid and they charge me fo cut ofs and re instating