unfair and annoyed

i have a joint account with two numbers on a mtn contract and , the last month mtn has made errors in the contract and charged me for service promos, my bill has been escalated to much more than i usually pay every month, i have called them on numerous occasions to have this restored and all i get is we will start an investigation process, they then suspended my lines due to non payment as i did not pay since i was awaiting final details, i made arrangements to pay the usual billing amount until its res loved and that was agreed upon, i now still receive messages that i need to pay the outstanding bill or my service will be suspended

my dilemma is that mtn has made the error with their promo charges and they have not correctly managed the billing, they cannot make it my problem by escalating by bill and forcing me to pay , they have stated that they have done this to numerous clients and i personally think its unfair and it be resolved as soon as possible, i would find it fair if mtn credits all this errors in billing and i resort to my normal billing amount, i have never been in arrears with mtn as records will prove,

i hope i have made my point heard and i find a suitable solution in this regard