Zte T230 User Manual page 27

There are two versions of the user interface that can appear depending
on the initial state of the handset. This is a common practice in
Android when there is two states that the handset can be in – Awake
and in use, or, asleep and idle (ie you leave the handset alone on the
kitchen bench).
The first scenario is you are already using the handset and it is awake
and active. In this case the previously displayed user interface will be
shown with the two buttons for Decline and Answer shown that you
would press to trigger the action.
The second scenario is when you have left the handset alone for longer
than it's display timeout period and the display has switched off. In
this case, there is another user interface that will be displayed for the
incoming call and it will require more then a press to answer or reject
the call.
This is due to the handset waking up with the incoming call on the
Locked Screen. To Answer, Decline or Decline with an SMS quick
response, you will need to swipe the action that you wish to be done.