Bad service from Nimble Group

My son was handed over to Nimble group for his cellphone account. In July we requested a statement for the outstanding balance it was send to us. He then paid the outstanding balance according to the statement that they send. Then when he finished paying the account he wanted a letter to state that the account has now been paid off. Then all off a sudden another balance appeared on his account. They cannot give us an explanation as why it was never incorporated in his original account and now they demand the amount to be paid. I just feel that they gave us a statement we paid it it is now settled. I have the proof to back me up and nobody gets back to me, we have send 3 mails already and no response, but they phone every week to ask when we are going to pay. I have even asked in the mails that the CEO contact me but to no avail.