Charged R500 for canceling within 3 days ..Telkom

Im very disappointed in the level of service received from telkom ..i took out a sim contract with sim with telkom and because the signal was terrible i cancelled in 3 days .i was told that i will only be billed Pro-rata but then got charged for cancellation bringing my bill to R500 how is this possible..i called in and was given attitude and placed on hold with no return ..im very disappointed and i want telkom to fix my bill and refund me my money...