To put a call on hold:
while speaking to the caller:
1) press the hold button

To hang-up on someone who is on hold:
1) press the "Cancel" Button, and the last person who you put on hold will be dropped

To resume a call which has been put on hold:
1a) press the line button corresponding to the call you which to resume

 

or
 

1b) press the "Resume" Button and the last call you put on hold will resume