Failed conversion to pre-paid

I phoned the Cell C Call Centre to convert my lapsed contract to pre-paid. Twice I went through the usual routine to be transferred to another department which would do the actual conversion. Twice I was disconnected on transfer due to non-availability of staff. The third time the transfer was successful with an unusual security question routine. My debit-order date is the 1st of the month but Cell C submitted the debit-orders on a fluctuation basis and Cell C argued that I got one question wrong. Previously, during the failed attempts I used the Contact Facility and submitted a written instruction to Cell C of my requirements. Now I am being harassed daily by Call Centre calls to do the conversion via the Call Centre only. Cell C will not accept my multiple email instructions to convert to pre-paid.