What is a "phone"? Is it a device with a rotary dial? Or is a device capable of making voice/video calls. A cellular iPad mini, IMO, is simply a mobile iOS device with a 7.9" screen. It can make/receive calls similarly to the 4" screen iOS device (think FaceTime audio, Skype, VoIP, etc.).
The fact that you must go and manually change it to your email address for outbound iMessages is the part that's invisible.