The duplicate SIM card isn't meant to work in tablets. It may in some instance work and provide data (though you'll find going forward that data won't be available in a tablet with this sim).
That said, you're iPad doesn't have a native dialer - it uses the facetime dialer when it's performing the "continuity" functionality, thus even it can't get phone calls natively.
So in this case 1 of 2 things is happening, either because the duplicate SIM is the tablet, the network could be getting confused OR you have calling on other devices (continuity) enabled between your iPhone and iPad.
This thread was the product of our fantastic collaborators during DIGITS Beta! As many terms have changed and bugs have been exterminated since the Beta launched, the information here may no longer be accurate. If you still have a question, please search our active threads or ask a new question in the DIGITS space!