Can’t get iPad Pro to answer calls when iphone rings

Start here:

Make a call with Wi-Fi Calling - Apple Support

Assuming that you also have an iPhone, to which your iPad “Calling Features” (and SMS Messaging) are linked, you may need to check/reset these features on your iPhone.

On iPhone:

Settings > Mobile Data > Wi-Fi Calling >

• WiFi Calling on This iPhone - set to ON
• WiFi Calling For Other Devices - set to ON

Settings > Mobile Data > Calls on Other Devices >

• Allow Calls on Other Devices - set to ON 
• [Allow Calls On] - verify that your iPad is explicitly enabled in the device list

On iPad:

Settings > FaceTime > Calls from iPhone > Calls From iPhone - set to ON

If prompted, enable WiFi Calling where supported by your Carrier

Finally, if all settings are correct and the issue persists, a forced restart of one or both devices may be necessary.