I made a mistake and picked voucher instead of cas...

I made a mistake and picked voucher instead of cash for selling my phone. I sent an email saying I made a mistake. They sent me an email saying they couldn t change it??????? Really. I have a voucher that I have no intention of using. I would not recommend quick mobile as this matter could have been resolved easily but they decided not to accommodate a genuine mistake. They have my phone now and won t return it. I did ask but got no reply. Very unhappy