CREDIT LIMIT ON TOP UP PACKAGE??

I am a Nashua Mobile customer who was transferred over to MTN, since this occurrence I have had endless problems with my account. My account is suspended every month and I have to phone in to unblock the line. I recently upgrade my contract from Anytime 500 to My choice 350 with the instruction to have the top up option. <br> Today again my line was suspended for the 3rd month in a row for exceeding the credit limit R1260 set BY MTN staff, when I requested them to increase the limit , the consultant Sibongile requested me to send him 3 months NASHUA MOBILE statements- FOR WHAT??? I have been with MTN since Dec and have always paid my account, why would they want to see NASHUA Mobile statements and how is it even possible reaching a credit limit when I should be on a TOP UP package?? I want a competent person to phone me as no one in the customer service centre understands my question.