Siemens optiPoint 500 entry Operating Instructions Manual page 16

Calling several persons simultaneously
Step by Step
ýFB
or
16 16 16 16
F<
or Enter the UK code or Standard code for conference.
If your telephone d oes not have a "conference" key:
Enter the code for conference.
A tone indicates that all three users have been intercon-
nected to form a conference circuit.
All three parties can now converse with one
another.