Is there any particular key set to press when you want to transfer a call to another extention number?
You would need to use SCCP for these features to work;
Check out Table 1 from this VG224 doc (show SCCP support for Conference);
Initiates a three-party conference.
During an active call, user presses hookflash for dial tone, dials a third party, and then presses hookflash again to connect all three parties
From this doc;
Hope this helps!