Polycom VVX 1500 C Business Media Phone for Cisco Unified Communications Manager (SIP) - Manual (Page 109)

Customizing Your Phone
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The following figure shows a phone set up with two lines (1952 and 1953) that
are each assigned to a Line Key, and a third line (1954) assigned to three
Line Keys. There is one available key that is set up as a Speed Dial Key.
Supporting Shared Lines
Your system administrator can set up your phone to support shared lines. If
two phones share a line, an incoming call to that number will cause both
phones to ring. The call can be answered on either phone, but not on both.
Calls on shared lines can be transferred, placed on hold, or added to a
conference.
If you have an incoming call on a shared line, you can tap the Silence soft key
to stop your phone from ringing. The call’s visual notification will still display.
Note Shared lines is an optional feature and may not be supported on your particular
system. For more information, contact your system administrator.
Note Call Forwarding is not available on shared lines.
Do Not Disturb (DND) on shared lines disables the ringing option only. You will still
see a visual notification.