Basically you should try:
AS5300(config-if)#isdn network-failure-cause 3
on the ISDN serial interface.
As per documentation this should work:
This command allows the original cause code to be changed to the value specified if the original cause code is NOT one of the following:
NORMAL_CLEARING (16)
USER_BUSY (17)
NO_USER_RESPONDING (18)
NO_USER_ANSWER (19)
NUMBER_CHANGED (22)
INVALID_NUMBER_FORMAT (28)
UNSPECIFIED_CAUSE (31)
UNASSIGNED_NUMBER (1)