Sometimes WiFi Calling will “fall-back” to reliance upon proximity of the iPhone instead of routing calls directly via WiFi and your Cellular Carrier via your internet connection.
Often all that is required is to restore full WiFi Calling from iPad Settings:
Settings > FaceTime > Calls from iPhone - check for a link to “upgrade” to WiFi Calling
In extremis, it may be necessary to first disable Calls on Other Devices on your iPhone - restart the iPhone - then re-enable the service first on iPhone, then the iPad.
On iPhone:
Settings > Mobile Data > Calls on Other Devices > Allow Calls on Other Devices - set OFF or ON as needed to resolve the issue
After cycling this setting, ensure that your iPad is specifically enable in the associated device list - and reset the FaceTime setting on your iPad.