Mitel telephone System Quick Reference Guide for 5212/5312 Handsets.

Answering a call  Lift the handset or press the flashing line key and begin speaking.
Making an external or internal call Lift the handset or dial 9 followed by the number.
Holding a call Press Hold key, the line key flashes.  The handset can be replaced.
Return to a held call To retrieve a call from Hold:
1 Lift the Handset
2 Press the flashing line key
Transfer a call Press the Transfer key.  Dial the extension number. Once answered announce the call if required and hang up.
To return to caller if the line is busy or no answer, press theCancel key.

Setup a conference call

 Establish your first call.  Press the Trans/Conf key.  Dial second number (internal or external) press Trans/Conf key to join all parties together.  Follow procedure to bring more calls in to the conference.  To leave the conference hang up or press the Cancel key.
Call Pickup Press this key to 'pick up' calls ringing at an unattended handset in your area.
To answer a call that is ringing at a station not in your pickup group: Press the Transfer key.  Dial the extension number. Once answered announce the call if required and hang up.
To return to caller if the line is busy or no answer, press the cancel key. 
Retrieve a message Lift the handset and press the Message key.  It flashes when a message is waiting.
Follow the Voicemail prompts.
Program a speed call Press Superkey press # until Personal Keys? appears. Press *  Press a Personal Key.  Press *  Dial the number.  Press the Personal Key again.  Press * to make the number private, or press # to keep the number visible.  Press Superkey to complete.
To call the Reception Dial "0."
Quick Reference guide for telephone icons Please take a look at the Handset guide here.