MTN won't give invoices so how can I pay them.

We were previously with Nashua Mobile for our premicell numbers. Without warning we were transferred over to MTN. The first we knew about this is when we received statements in the post from MTN. We have been trying to source invoices from MTN since December 2014 without any success. The numbers just go to music for half an hour then we give up. We tried to speak to the call center again on 5 February 2015 but after entering the cell number as usual the system said we had been handed over for non-payment. We did eventually get through to a call center agent (Lesego Ramela), on 20 February 2015 only to be advised that \we don't have the tax invoice