Double debits

Good day . I received an sms saying that an amount was going to be debited on my account . According to my knowledge the amount they want is not what I must pay. Even though I have proof of all my debits. I cancelled one contract on the 3rd July then took out another cheaper one just to keep my no as it’s a sim only contract . When it came to the 1st August my contract bill was still the same although my new contract was only R29 . And it was done before the 15th of the month . So how come my bill was still high and how come they get an amount which is not right