Charge you for the period before you even signed the contract

I took out a mobile contract on the 21st of April. I noticed my debit order amount was double to what it should be. when chatting to a consultant and later the supervisor, I was told that TELKOM charges you the FULL MONTH for the subscription fee, irrespective of whether you only signed up on the 21st of the month or the last day of the month. how can I be billed from the 1st of April, when I was not even a client of TELKOM, and 2ndly when I did not even have the phone in my possession ???? And they refuse to credit me for the over-charge as they insist their billing is correct.