ok i just did a test, CorsixTH running, made a call to the phone with keyboard open, sure enough the phone call popped up, slid the keyboard down and answered in 5 seconds.

then i minimized corsix and started watching a video (with Corsix in the BG), repeated the test and recieved the call and answered (insluding switching from lscape to portrait) and it answered within 5 secs.

You have a faulty device.