Billing after contract ended

Towards the end of 2017 I have queried how long I still need to pay for a specific contract and wanted to cancel as I was not using the device at the time anymore. The call center operator told me that it will expire at the end of December and it is better not to cancel. She said I should leave it to expire and I won't be billed anymore. However come the 9th of January 2018 I hot a new month's bill. I phoned the call center once again and they put in a termination on my account. But I still need to pay for February even though we are at the start of January now and February is still far away. Apparently it will be processed only in Feb. They are trying to suck every last cent out of their customers. I have a good hunch that March I will get another bill.