iPad ringing persists after iPhone accept calls
First, let's remove the iPad as a device that is allowed to receive calls from your iPhone. 


  On your iPhone, go to Settings > Phone > Calls on Other Devices, then
  turn off Allow Calls on Other Devices for that iPad. 
  
  On your iPad, go to Settings > FaceTime, then turn off Calls from iPhone.


Then restart both devices. And then turn on "Allow Calls on Other Devices" for that iPad as well as turning back on Calls from iPhone on the iPad.

Now retry receiving a call and answering it on the iPhone. Make sure both ring, but see if the iPad stops ringing when you pick up the call on the iPhone.

If the issue persists, check the software version for both devices and make sure they are on iOS 12.4.1 as of this post. If so I recommend backing up the iPad before continuing to the next step.

Next let's try Reset All Settings on the iPad (DO NOT ERASE iPad). 

Reset All Settings sets to default all of your preferences and settings within the Settings app. This is why I recommend backing up the iPad before doing this.