The Asterisk project reports:
If Asterisk is challenged on an outbound INVITE and
the nonce is changed in each response, Asterisk will
continually send INVITEs in a loop. This causes Asterisk
to consume more and more memory since the transaction
will never terminate (even if the call is hung up),
ultimately leading to a restart or shutdown of Asterisk.
Outbound authentication must be configured on the endpoint
for this to occur.