Panasonic KX-TD500 System Reference Manual page 240

(2) What if a call ringing at overflow destination
is not answered?
In case the overflow destination is an
attendant console or extension.
1. When the call ringing at overflow
destination has originally arrived to a UCD
group by call transfer.
(Transfer Recall)
The call will ring back at the attendant
console/extension who transferred it, if not
answered until the transfer recall timer has
elapsed.
CO
ATT/EXT.
Transfer
UCD group (05-32)
(All Login)
Overflow destination (ATT/EXT.)
No Answer (Transfer recall timer
2. When the call ringing at overflow
destination has originally routed via DISA.
<Disconnection>
The call will be disconnected automatically,
if not answered within 60 seconds.
5. Auto-Logout
When an extension in a UCD group does not
answer more than a set time period*, the call
will be automatically transferred to another
member's extension.
If "No Answer Time-Out" occurs twice in
succession, the extension is automatically set
to logout status.
This feature is disabled by the dumb
command "UCD Auto Logout Operation
(ULO)".
* "Call Forwarding-No Answer Time-Out"
has elapsed)
3-D-22
Operation
Making a call from extension to a UCD group
1. Lift the handset or press the SP-
PHONE button.
• Dial tone 1, 3, or 4 sounds.
2. Dial FDN of the UCD group.