Thank you @Flohack for your reply.
I think that is not a problem of Ubuntu Touch. Unfortunately it is related to the touch screen of my Nexus 4. I installed a paint app on UB Ports and check my touch screen and found out the down part of the screen where the Call button is located does not work properly. To do a more investigation, I also installed the Official Rom 4.2.2 and did the same check and the problem exists.
Thank you.