Billing you a prorata ontop of normal bill after being told you wont be billed the prorata

So today i receive a statement from Supersonic and i decide to have a look at i(as i had a small balance that was overdue as a result of being unemployed for a while in 2020) and i notice i was billed for an amount of R2245 in the month of April. I decide to call and query this to be met by an unprofessional and not helpful agent who checks for me only to state that because of my upgrade that went live in March 2021 i was billed prorata. I then questioned the fact that the agent whom assisted me with the upgrade told me that i wont be billed on prorata and she simply took a few moments of silence just to come back and tell me that the employee whom told me this does not work there anymore and cannot be held accountable and therefor i need to pay the amount. At that time the employee represented supersonic and he told me something that i am sure of is recorded on their system and therefor is a legal binding agreement and should be honored by supersonic irrespective if the employee still works there or not. I explained this to her just to hear her bluntly they will not honor this as the employee is no longer working there and i must pay the amount. After questioning this again regarding the recording and that no matter if he is there or not they should still honor what he said during his employment, she told me to hold on and after about 3 min still not even yet returned, i proceeded to hang up and she didn't even call me back with feedback of whatever she went to do in that time while i was holding on for her. I do not like being here on hellopeter everytime,but it seems it is the only why for management to get involved and to get issues sorted as the staff over the telephone are useless. I do hope to hear from a manager A.S.A.P