Bad service

An amount was deducted on my account by a reference connect signup top up, i took out a phone with them but i paid it up last month and now an amount has been deducted, I called in i spoke to an agent who told me that I should’ve cancelled the contract after the last month but I was informed that when i took it out so im a bit confused why a contract cant be cancelled automatically after i have paid it up cause i dont owe them anything