There are translucent buttons next to the LCD on the phone. These buttons light up green when you are on a call.
If you are on a call and have another call coming in, you can press the illuminated green button to simultaneously put your current call on hold and pick up the next call.
You can toggle between the calls by hitting the green illuminated button again.
If you are on a call you can press the same illuminated green button to put that call on hold and open up another line to dial out another call.
If you have 2 separate calls on on the phone at one time you can join them. One of the soft keys (buttons below the LCD) will appear with a label of "Conf" You can hit this button to join the calls into a conference call.