Panasonic KX-TDA0484 Programming Manual page 38

2.2 Programming
The automatic route redirection feature activates in this configuration. If a call is made through card
A to gateway group 1 when all 4 channels of card B are busy, card A automatically redirects the call
to card C.
This is possible because by grouping, PBX 1 sees PBX 2 as having a combined set of 8 channels,
not 2 separate sets of 4 channels.
Note
The automatic route redirection feature cannot be used in a network where a gatekeeper is used.
For details about gatekeeper settings, refer to "Gatekeeper Settings" in "2.2.2 H.323
Parameters".
38 Programming Guide