Polycom SOUNDPOINT IP 450 User Manual page 40

User Guide for the SoundPoint IP 450 Desktop Phone
Note
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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 both.
Calls on shared lines can be transferred, put on hold, or added to a conference.
Call Forwarding is not available on shared lines.
Do Not Disturb (DND) on shared lines disables the ringing option only, the visual
indication is still in place.