I ultimately fixed it by flashing the Mi10T with the newest Xiaomi image. Then I followed the procedure to put the (in my case the MicroG appolon LineageOS) image again on the device. After that, the phone calls worked, incoming as well as outgoing.
I went for LineageOS since the image was a bit newer and I wanted to test that the newest version solved the problem. Initially (with the off-fabric original Xiaomi image) it behaved the same as /e/-os. After the above procedure I got it working. I recon that /e/ will behave identically, but in my case I stayed with LineageOS and did some de-googling myself.
Note, that in case you have (like me) multiple SIMs in your device, the default SMS app did not work like expected (could not select the SIM to use, could not send any SMS with it), but changing that app to SMS Messenger solved that problem.