Ericsson 50 - Manual (Page 13)

OUTGOING CALLS
11
BusinessPhone 250 / BusinessPhone 50
Standard Telephone / Economyplus Telephone
BUSY EXTENSION You call an extension and receive busy tone.
Camp-on You can notify a busy extension of your call by a muted
ringing call (if this function is allowed).
Press to camp on
Keep the handset off hook. When the called extension
replaces the handset, it will be called automatically.
Note: If you receive the busy tone again, the desired extension
does not allow camp-on.
Intrusion You can intrude on an ongoing call on a busy extension
(if this function is allowed).
Press to intrude
Intrusion tone is heard and a three-party call is established.
When the called extension replaces the handset and you keep
off hook, the called extension will be recalled automatically.
Note: If you still hear the busy tone, then your extension does not
have the authority to intrude or the desired extension is protected
against intrusion.