Poor Service and Misleading Sales Agents

I had a contract with cellphone contact with Vodacom, which ended in May this year and one of your agents called me to offer me a new cellphone contract which I declined as at the time, I didn't want a new phone. He then offered me a Flex 65 plan for which I will get R65 airtime every month with a like a R10 discount which I accepted. I was told that I would be able to add a device on this contract when I wanted to upgrade. Today I called Vodacom sales and now I am told that I can't add a device on my plan and will need to apply for a new line. Which means that I get another cellphone number that I don't need. Had I been told that if I accept this offer I will not be able to get a device on my plan, I would have not accepted this offer because I knew I was be looking to out the new device within the next 6 months, now I am told I will need to take out a new plan on top of my existing plan if I want a new device. When the sales agent told me that I can add a new device to my existing plan!