You can transfer calls to any other extension of your phone system, or to any 10-digit USA/Canada number.  If you need to transfer your call, but wish to first speak with the person to whom you are transferring the call, you may use the 'attended transfer" option.  

When you are on a call, your in-call display will change to this:

If you wish to do an "Attended Transfer," press the "att. transfer" button, and dial the number where you wish to send the call.  Stay on the line, speak with the party, and hang up to send the call. 

If the other party does not wish to take the call, they can hang up, and you will go back to your original phone call.