Vodacom dont listen to customers requests.

I received a call from Vodacom offering me a phone on contract. I specified the device and ALSO specified that I want it on a Vodacom prepaid number which was previously on somebody else' vodacom contract (cancelled 6 months ago). I got a "please confirm" sms, to which I rep**** "change options". However, the courier simply delivered last week. Phoning them today, they say that we have to return the device via courier and then cancel the contract and then take out a new contract. I offered to bring the device to Midrand and do the necessary paperwork, but "that cannot be done". It has to be returned via courier. Why? and why can't they just change the number?