MTN -Shocking Client Service

I am a medical Doctor & MTN client on contract since 1994.I upgraded my contract in March 2020 & was promised 300min & 2.5g . Unfortunately my call minutes have been disappearing without me making any calls. I have been to Gateway Umhlanga store several times and they have been constantly logging the complaint & have asked me to call 135 for update on my problem. To date the problem remains unresolved . I find this unacceptable & frustrating as i have had to constantly buy more airtime which surprisingly does not disappear.The worst part is noone is accountable & able to resolve!