Oh Vodacom

I am due to a upgrade in march 2018 i phoned vodacom an got the deals they can give me. Thinking in the times we are living and try to save i phoned Cell C and save R 500 per month with a better phone witch i am over the moon ! So now i phone Vodacom to cancel my contract with them ! They told me i am due for a upgrade but can only cancel this contract in June so i asked the operator if i want to cancel now what would be the cost and he awnsers me that i can only phone them back after the 4th ! I mean why do i have to phone them back ? Or what could be so difficult to cancel ?