Sharing an AppleID between multiple users is always a very bad idea - as doing so can cause all kinds of issues. An AppleID is intended to be used by a single person.
This issue aside, if I understand your problem correctly, your problem is this...
When at home and connected to the same WiFi network, you can make and receive phone calls from your iPad (using the FaceTime App) and your iPhone. However, when away from home - and not connected to WiFi - whilst you can make an receive calls from your iPhone, you are unable to do so from your iPad. This is expected behaviour.
On its own, your iPad cannot make or receive calls over a Cellular Network - whether or not it is a Cellular model of iPad.
If your iPhone Cellular Calling plan both supports and includes WiFi Calling, then if correctly configured on your iPhone, your iPad can access Cellular Calling features of your iPhone whilst connected to a WiFi Network.
Here are the Apple support pages that describe the Cellular Voice and Messaging (SMS/MMS) features that can be accessed from iPad when associated with an iPhone - and the Continuity Services that must be enabled:
Continuity
https://support.apple.com/en-gb/HT204681
SMS/MMS Messaging
https://support.apple.com/en-us/HT208386
WiFi Calling
https://support.apple.com/en-gb/HT203032
When correctly configured, WiFi Calling can allow an iPad to independently access the iPhone Calling plan over a WiFi connection. Once enabled and configured - assuming that the Cellular Carrier plan permits these features, the iPad can still access shared Cellular Voice and Messaging features whilst the associated iPhone is turned-off - or otherwise elsewhere - but only whilst the iPad has a WiFi network connection.
I hope this information and guidance is helpful in both understanding and configuring the necessary services.