On the Sipura phones:

Line Key 1 is a  private extension
Line Key 2 will be shared extension (this is 21 in this example)

SAMPLE SPA 942 CONFIGURATION

Sip Tab
	SIP-B enable: NO

Phone Tab
	Line Key 2; Extension: 2
	Line Key 2; Share Call Appearance: shared
	Miscellaneous Line Key Settings; SCA Line ID Mapping: Horizontal First
	Miscellaneous Line Key Settings; SCA BARGE-IN ENABLE: YES
	Line Key LED Pattern: Remote Seized LED: c=a
	Line Key LED Pattern: Remote Held LED: c=a;p=s

Ext 2 Tab
	Share Ext: shared
	Shared User ID: 21