If you're using Uverse VOIP service, try logging in through att.com/myatt using your member ID. Once logged in, follow the below click path:
My Digital Phone > Check or manage voicemail & features * > Phone Features > Call Waiting > Then turn off.
* Sometimes, after clicking "Check or manage voicemail & features," it will ask you to log in over and over again. If this happens, depending on what browser you're using, try accessing the portal through Private Browsing / In Cognito mode (Ctrl + Shift + P or N) or you can also try clearing out your brrowsing history including the passwords (Ctrl + Shift + Del).