Giant Electronics Ltd 2G4DSSL2400 ユーザーズマニュアル

In order to carry out internal calls, more than one handset must be registered on the same base station.
1) Press the key.

2) Press the number of the required handset or ‘*’ key to page all handsets.

1...4  *
OR
Press the UP/DOWN key to select the desired handset(s) in the menu, then press PHONE or PROG.

,   

 / 

3) End the call by placing the handset in the cradle or pressing the PHONE key.

Note: If an external call is received during an internal call, a short acoustic signal is issued.

If a portable is engaged in an external connection, it is possible to put the external call on hold and establish an internal call in the
same way as the previous section ‘Making internal calls’.
Then the display will become as below if handset #1 is selected.

TALK 1:35:06
HANDSET #1 H

The 'H' in the right hand side of the third line indicates having the external call on hold.
It is possible to toggle between an internal call and external call by pressing the key.