Vodacom error

I went to vodacom in menlyn mall pretoria to get a new fone for my autistic son and I got sold a fone on a specific premium per month which I signed for, the consultant made and error and only realized this once I left with the fone and gave it to my son now, they whould like me to pay extra or return the fone which I think after I've signed is no longer my problem

Please see email recieved as follows, I can provide Email on request as I cannot add it to this review but this is terrible service and I refuse to be charged more or to change the fone as the error lies with the company and the consultant that helped me her name is Amanda