I'm not 100% sure that a SIM Card can dictate settings on a phone ... At least it can't on my mobile.
I have a Samsung phone with a Giffgaff SIM Card, and tbh, I had to look up how to activate Call Waiting, Call Forwarding, & Call Barring ... But once I found what I was looking for it allowed me to make any changes I wanted to.
Can you actually get to the Call Waiting slider on the phone, or will it not let you reach that far?
Sorry to hear you've had some issues with call waiting. If it is showing as active on the handset and it's still not working then we can check the settings from our side are all correct too - just get back to me via PM and we'll take it from there.