Zte T230 User Manual page 26

 Dial the number, and when connected, tap Merge
MAKING AN INTERNATIONAL CALL
 Press the Call Key
 Hold down the 0 key to enter the '+' symbol for International Dialling
 Enter the full phone number.
 Tap
 Press End to end the call.
ANSWER AN INCOMING CALL
An incoming call presents three options:
Press
Press
on the home screen to open the dialer.
to make the call.
to answer the call
to reject the call.