Incorrect Debit Date

I took out contract on 01/08/2014 and requested debit order date of 20th of each month. However, in December 2014 I went to Cell C store in Liberty Promenade Mall in Mitchells Plain and requested them to change date to 31st of the month and they informed me it has been done but they cannot do it for December but will be active from January<br> On 20/01/2015 I receive SMS of debit order deduction which was returned unpaid as no funds in account. i then went to the store the next day and was informed but debit order date was changed. i then informed me them of sms and couldnt explain what happened. Same thing happened in February as well March and on all 3 occassions it cost bank charges and negative profile at bank. In March my line was cut due to non payment before the 31 eventhough i requested change of payment date and when speaking to customer services was informed problem was fixed but on 20/04 they requested debit order again and also requested more than contract amount.<br> So contract is costing more money then agreed upon due to all the unpaid bank charges and i consider this as theft as they dont have permission to access my bank account on 20th of the month.