MTN service is up to S**T

MTN service is up to s**t, since the changed there systems we have been having problems with my services being suspended every single month. I have been paying cash for my contracts for more than 5 years and at the end of each month. Now with their new system said payments needs to be made 21 days from invoice date. I emailed them regarding this matter how many times and the problem is not resolved, to be said I do not even get feedback from any person emailed. Said persons is Rhulane Mashaba, Bonginkosi Mafa and Xolile Dlamini, the first 2 names does not even read my emails, all emails is delivered to them. How can a system change on their side change a said agreement that has been in place for more than 5 years with out speaking to me or notifying me that the changes will affect me in the following matter.