These directions are for the KX-TG5451, KX-TG5452 and KX-TG5453.
If you want to use the intercom feature, you need to know the handset number for the unit you want to contact. The base is zero. The handset number is set when you register the handsets. So to communicate with the base, press Intercom on the handset followed 0. The base will ring until someone presses Call to answer it (or one minute whichever comes first). (Alternatively, you can use the up/down arrows to select H1, H2, H3 or Base. Then press the right arrow key.)
To communicate between two handsets, press Intercom and then 2 (from handset 1 or 3 to connect with handset 2). Again the handset will ring for one minute or until the person with handset 2 answers. (To answer, they can press the phone, speaker or intercom button.)
If someone at the base wants to use the intercom, they need to press Select until the appropriate handset number is displayed. Then press Call.
To transfer a call, press Intercom and then select the desired handset. When the intercom message is picked up, indicate that a call is being transferred and press Off at the handset or base that initially answered the call.
For a conference call, initiate the call then press Intercom and select the handset to join the conversation. Once that person picks up, then press Conf. (I believe that Conf is a softkey so check the display.) This is only possible with the handset. (Another handset can join the conference by pressing the phone or speaker button on that unit.)
The manual for the KXTG5453 is available here: http://service.us.panasonic.com/OPERMANPDF/KXTG5451S.PDF
If you have a different model of the Panasonic 5.8 GHz phone, please add a comment with the model number and I'll try to give the directions for that unit.
I hope this helps.