billed for no service

I bought a cellphone on contract at East London Hemmingways branch on 20 May and took it back on 22 May 2015 due to network inavailabilily on my device that I experienced.After a long argument the salesladymanager agreed to give me cancellation forms,I signed and left.A week later I received a bill of R1 600.00 and my contract has never been cancelled.I dont know what the bill is about.Whenever I phone to try and understand they keep on passing the buck. Pls hlp me.