AT&T System 25 - Manual (Page 67)

Transfer
You can easily transfer calls to another System 25 telephone.
You can also transfer some calls to outside lines. However, you
should see your System Administrator for more information
about this type of transfer.
When you transfer a call, the original person is temporarily
placed on hold and is not connected to the other person until
you hang up. At that time, the call disappears from your
telephone.
Transferring Calls
The light next to the button of the call on hold blinks.
2 Dial the other person's extension or press a line button, such
as
NOTE: If you misdial the number, return to the person on
hold by pressing the button next to the blinking light. Then
return to Step 1 and continue.
3 Hang up if you do not want to announce the call.
Otherwise, announce the call and then hang up to complete
the transfer.
NOTE: If the person you attempt to call does not answer,
press the button next to the blinking light to return to the call
on hold.