Wrong billing

We have a WiFi with telkom February /March the WiFi is a 4meg line uncapped for R599 a month with instalment included and the first (1) month R1100+- and second (2)month R604+- and (3) month is R604+-and R600+- and we pay R600 again . We pay every month with a debit order it is showing on our bank statements that is proof we had pay it and now they say we thit not pay for two months and they say we need to pay is before they can do anything for us laas week it was a R1100 +-and now its R2200+- in a week . I really need help with this problem please.