Contract upgraded on the incorrect line and incurring the customer with extra expense!!

I am so disappointed with Vodacom yet again. Ive always been a loyal customer to them..On the 18th i upgraded my contract on my main line and the agent asked me to confirm my number which i did. I received my new device and thought everything was fine. This morning i get an sms to say that the upgrade has been complete but the number they had on the sms is my sons number. When i called the call center, they said obviously there is a mistake but they can help by switching me to a sim only deal. But i know need to pay extra handset fee for my sons line which i didnt ask to be upgraded. Why am i now paying extra because vodacom's agent messed this up.