Disable Call Waiting
So I've discovered there is a way of doing this.
I found this answer on Quora:

https://www.quora.com/Where-is-the-call-waiting-option-on-Android-7

There's a video there and the answer is essentially :


Open the phone app Press the Menu button (3 dots top right) 
Press   Settings
Press Calling Accounts 
Press on your call Operator (Was EE in my case)
Press Additional Settings 
Slide Call   Waiting to disabled