Most cordless phones will have a "Flash" button, or if they have separate "Talk" and "Off" buttons, pressing Talk should generate a Flash.
If you are on a call, pressing Flash, dialing an extension number, and then disconnecting will transfer the call.
Pressing Flash, and dialing your OWN extension number, and then disconnecting will PARK your call. To get it back, from the cordless or any other phone, dial Intercom, 6, and YOUR extension number. (The cordless extension should be programmed to select Intercom first, and then dial 9 to access outside lines.)