25 payments required to end a 24 month contract

So I started a post paid Pinnacle contract in February 2018 for 24 months +- R1000 per month - at the time excellent value for me. In March 2018, I was double billed for February 2018 and March 2018 - and thereafter every month. In December 2019, I contacted CellC and advised that I want to migrate onto the 66/min prepaid - which I think is also excellent value. As a 68yo, my cell phone needs have changed Today I receive my normal bill of +-R1000 from Cell C. So I phoned customer care and retentions and they explain to me this is correct because it is for January usage. However, when I was billed in March 2018 for February 2018 as well, being a simple minded guy, I count now 25 payments to end this 24 month contract. If someone can explain to me what I am missing, I would appreciate it.