Call waiting is a useful tool that is included on most phones that allows you to place one call on hold while you answer an incoming call. Windows Phone 8.1 comes with this feature, and to use it, all that you have to do is enable it in the phone's settings.
This walkthrough will show you how to turn on the call waiting feature on your Windows Phone 8.1.
Activate Call Waiting on Windows Phone 8.1
Go to App List > Settings. Go to the bottom of the menu, and tap network+:
Under Call settings, tap Set :
Now move the Call waiting switch into the On position: