Comment 0 for bug 1493574

A similar, if not the same bug, has been reported to be fixed for OTA6. This bug report explains that it's no necessary to wait for a minute or so on the phone call, but the screen turns black immediately, and doesn't turn back on again to allow hanging up via a screen control.

Potential duplicate: https://bugs.launchpad.net/canonical-devices-system-image/+bug/1483127

Device: Aquaris E5 HD Ubuntu Edition
OS version: 15.04 (r5)

A) Incoming phone call:
       - Accept the incoming call by sliding the grey middle button to the green button
       - Watch the screen turning black
       - Bring the phone to your ear, speak (as in a normal phone call)
       - Take the phone away from your ear, note that the screen stays black

       How to hang up anyway: (workaround)
       - (continued from above, screen stays black after taking it away from your ear)
       - Press the screen lock/unlock button (= button above volume control)
       - Screen with phone app shows for about half a second, then turns black again
       - Press the screen lock/unlock button again, and try to press the red hangup button quickly
       - Repeat the previous step until hanging up succeeds

B) Outbound phone call:
       - Open the phone app
       - Slide up Recent calls list from bottom edge, select a phone number
       - Press green dial button to initiate the call
       - Watch the phone dialling and the screen turning black
       - Bring the phone to your ear, speak (as in a normal phone call)
       - Take the phone away from your ear, note that the screen stays black

       How to hang up anyway: (workaround)
       - (same procedure as in A. above)
       - Difference in behavior:
         * In some calls after pressing the lock/unlock button for the first time the screen stays alight (doesn't turn black again after half a second).