Why has my iPad stopped receiving phone calls?

Assuming that both your iPad and iPhone are signed-in to iCloud with the same AppleID, verify your iPhone and iPad “Calling” settings:

On the iPhone

Settings > Mobile Data >

• WiFi Calling >
  • WiFi Calling on This iPhone - set to ON
  • Add WiFi Calling For Other Devices - set to ON
• Calls on Other Devices
  • Allow Calls on Other Devices - set to ON
  • [Allow Calls On] - explicitly enable your iPad from the list of devices

On the iPad

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

If all settings are as above, force restart both the iPhone and iPad:

Force restart iPhone - Apple Support

Force restart iPad - Apple Support

See this note

* Don't see the Text Message Forwarding setting on your iPhone? Go to Settings > Messages, turn off iMessage, turn it back on, tap Send & Receive, tap Use Your Apple ID for iMessage, then sign in with the same Apple ID used on your other devices.

in

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support