See the following from the Cellular Calls section of Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support
Use iPhone Cellular Calls with any Mac, iPhone, iPad, or iPod touch that meets the Continuity system requirements. It works when your devices are near each other and set up as follows:
- Each device is signed in to iCloud with the same Apple ID.
- Each device is signed in to FaceTime with the same Apple ID.
- Each device has Wi-Fi turned on.
- Each device is connected to the same network using Wi-Fi or Ethernet.
- On iPhone, go to Settings > Phone > Calls on Other Devices, then turn on Allow Calls on Other Devices.
- On iPad or iPod touch, go to Settings > FaceTime, then turn on Calls from iPhone.
- On Mac, open the FaceTime app, then choose FaceTime > Preferences. Click Settings, then select Calls From iPhone.
Turn off Settings > Phone > Calls on Other Devices > Allow Calls on Other Devices.
I would also suggest that you carefully consider not using the same Apple ID. They are not designed to be used by more than one person. You can (and should) each have your own and you can still share selected things via Family Sharing Family Sharing - Apple Support