Panasonic KX-TD500 System Reference Manual page 1203

(4) SLT On-hook Operation Mode
In single line telephone procedures, active call is put on consultation hold
when the switchhook is pressed down for approximately 1/2 second and
released.
In this case, consultation hold recall tone will ring immediately if you replace
the handset on the switchhook without dialing any digits.
Then you may hear recorder tone when you lift the handset to reply this
ringing
This may happen sometimes if the handset is replaced on the switchhook
after hopping on it.
To prevent such unexpected consultation hold tone from ringing, select "2"
for this setting.
When setting "2" is selected,
a call put consultation hold will be disconnected if you replace the handset
on the switchhook without dialing any digits.
When "1" is selected. — default
(1) Talking
Talking with a
caller.
When "2" is selected
(1) Talking
Talking with a
caller.
(Note)
To hang up and make another call right away, an SLT user should be sure to
hold down the switchhook for more than two seconds.
(Parameters)
1 : Hang-up causes ringing of consultation hold recall tone. (default)
2 : Hang-up disconnects a call on consultation hold.
(2) Hooking (3) On-hook
A call is put on
consultation
hold.
(2) Hooking (3) On-hook
A call is put on
consultation
hold.
11-C-77
(4) Recall
Without dialing Consultation hold
any digits. tone rings.
(4) Disconnection
Without dialing A call on
any digits. consultation hold
is disconnected.
RING!
RING!