If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages (green bubbles) from your iPad you must explicitly enable Text Message Forwarding for your iPad on your iPhone...
On your iPhone:
Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON
As for answering and making calls from your iPad, check these settings:
On your iPhone:
Settings > Phone > Calls on Other Devices > Allow Calls on Other Devices - set to ON - and explicitly enable your iPad from the list of devices.
On your iPad:
Settings > FaceTime > Calls from iPhone > Calls from iPhone - set to ON