CANCELLATION NOT ACCEPTED

A agent call me regarding a funereal plan and told me I can cancel if I am not happy with the policy. The policy was issued 19 August 2015 with the agreement that the 1st installment will go of after 3 months to give me time to go through the policy. I received the document in the post 7 December 2015. I immediately went through it, was not happy and let them know I want to cancel. They have by then deducted 4 premiums. An agent call me on 23rd December to tell me I wont get my money back because I did not cancel within 3 months. How can I cancel before I received the documents?????