This phone uses virtual intercom/call appearance buttons. Each such button assigned to your phone by the system administrator represents a call that you can make or answer. While you can only be connected to one call at any time, you can easily switch between it and your other virtual call appearances by using the 
Hold button.
Typically the maximum number of simultaneous calls allowed is 3. This is set by your system administrator. When all your virtual call appearances are in use, you are busy to any further calls. Those calls will follow your forward on busy if set, else go to voicemail is available or else just get busy indication.
Virtual Call Appearance Usage
When you go off-hook by lifting the handset, you answer the alerting call if any, otherwise you hear dial tone and can make a call. This uses one of your virtual call appearance.
With a call connected:
•If another call arrives on another virtual call appearance, you will hear a call waiting tone. Your phone's display will switch between details of the current and the waiting caller.
•If you press Hold, the connected call is placed on hold and one of the following occurs, in the order listed.
1.If you have a call waiting, that call is answered.
2.If you have any free virtual call appearances available, you will hear dial tone. This allows you to make a call or to use short codes that may affect the held or waiting calls.
3.If you have a call on hold, that call is reconnected.
•You can press Hold again to cycle through the options above.
•If you press 
Release or replace the handset (go on-hook) during a call, the current call is ended and your phone returns to idle. If there is a waiting call, it starts ringing. You can answer the call by lifting the handset again or pressing Hold.
With the phone idle:
When you lift the handset, the following may occur in the order listed:
•The first alerting call appearance is answered if any.
•The first free call appearance available is seized and you hear dial tone to make a call.
•You can press Hold to switch between virtual call appearances. This will answer or retrieve any call on next virtual call appearance or else play dial tone to make a call.
With the phone idle but a call alerting:
•Going off-hook or pressing Hold will answer the call.