PYLE Audio SOUNDPOINT IP 450 - Manual (Page 40)

User Guide for the SoundPoint IP 450 Desktop Phone
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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.
Note 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.