Is anyone actually able to make/receive real, regular cell
phone calls using linux on the Gemini?
If so, does it actually compare to a "normal" smartphone
calling user interface experience?
The answer is yes. But it doesn't out of the box with debian. Let me explain - sailfish can make/receive calls/sms etc. Sailfish is a vanilla linux distro with a fancy front end. In fact, it shares the kernel with debian - sailfish is a "stowaway" (check out the .stowaway directory in debian). So if sailfish can do it, so can debian. The problem is that in debian it isn't set up by default. You would use ofono I believe. In the real world, I don't know if anyone has got it working in debian.
As far as the "normal" smartphone experience, in as much as the gemini can do it, sailfish behaves as you would expect a smartphone to. I use it everyday, and have since it was released for the gemini. I answer calls with a swipe, finish by clicking end, ignore with a swipe etc. Phone calls interrupt what I am doing (ie display the incoming phone call screen). It has a phonebook, recent call history etc. You can use it hands free, connect to your car or use BT headphones. Basically no different to android.
The one difference is that you have to open the gemini to see who is calling. Yes, you can install apps which flash the LEDs, but I don't use these. You can pick calls up via the silver button (same as android) without looking at who is calling. With the cosmo, the second screen will let you behave more like a conventional phone - you won't have to decipher an LED pattern, or open it up to see who is calling. There is a rocker switch to accept/decline calls, so all in all, it will be a standard smartphone.
For today, if you want linux and phone functionality, use sailfish. If you want to use debian, you are probably out of luck.
Hope this is what you were looking for.
PS. as sailfish is a full linux distro, you can pretty much do anything you could on debian except run X11 apps (sailfish uses wayland). I have installed a full development environment which I use through the terminal. I can ssh/wget/ftp etc the same as any linux. I can install packages/libraries with pkgcon add/remove, the same (but different command) as any linux. It runs SDL/Qt/QML apps.....well, you get the picture.