Vodacom does injustice to me

After the salesman making me sign a phone contract without informing me that l was signing the official contract as a first time phone applicant,an hour later l returned a contract phone that l was not happy about. The sales person and the manager told me that l can not return the phone back, after two days of this matter, the phone was finally in their possession but they have refused to cancel the contract and l have a pending debit order in my account. Please help me, l have tried so many ways to get assistance but l haven't been helped.