Hi, I had this problem, too, and I am not sure if this has anything to do with the proximity sensor. I just rebooted my FP2 and now it’s working! The proximity sensor works like it should do.
If reboot doesn’t work for you, there is at least a small workaround so you don’t have to wait for the other caller to end the call: Go to Settings --> Accessibility and activate “Power button ends calls”. Now you can use the Power button to end calls when you need to.