Bad experience

I have a contract with Telkom with the agrrement that they will deduct from my bank on the 25th of every month, so what happened is that last month they debited on the 25 and they claimed that they did not get any money from my account hence my salary deposited on the very same date. On the 16 of february they tried to debit again and there was enough money left, now they have blocked my number and they say i must pay R670.00 from R199.00. i am really not happy.